Learn to express yourself like an ornithologist with this expanded glossary of terms.


abdominal air sacs
A pair of air sacs in the abdominal region of birds that may have connections into the bones of the pelvis and femur; their position within the abdominal cavity may shift during the day to maintain the bird’s streamlined shape during digestion and egg laying.
abducent nerve
The sixth cranial nerve; it stimulates a muscle of the eyeball and two skeletal muscles that move the nictitating membrane across the eyeball.
Nonliving; includes both things that are dead (such as dead leaves) and those that have never been alive (for example, rocks).
abiotic factors
Features of the environment that result from non-living physical attributes such as temperature, topography, rainfall, altitude, etc.
accessory nerve
The eleventh cranial nerve; it carries motor output to constrict the neck muscles.
Changes in a bird’s physiology after it has been in a new environment for several days or weeks that allow it to function better in the new conditions.
The changes in the curvature of the lens (and cornea, in birds) of the eye brought about by the action of the ciliary muscles. These changes allow the eye to focus on objects at different distances.
At the hip joint, the hollow on the pelvic girdle into which the head of the femur fits.
acid rain
Rainfall that is unnaturally acidic owing to atmospheric pollution.
acoustic nerve
See vestibulocochlear nerve.
A genetically controlled trait that increases an individual’s fitness relative to that of other individuals. All kinds of traits—from subtle aspects of internal physiology to complicated avian behaviors—are potentially adaptations.
Describes a trait that better promotes an individual’s fitness than does some alternative form of that characteristic.
adaptive (or acquired) immunity
The components of the immune system that become tailored to respond to previously encountered pathogens, usually involving antibodies that recognize and attack specific antigens on those pathogens.
adaptive management
A type of ecosystem management (see separate entry) in which managers continue to learn more about the ecosystem as they proceed, and continually modify their management techniques to incorporate the new information.
adaptive radiation
The evolution, from a common ancestor, of a variety of different species adapted to different niches; the species usually have different morphologies and behaviors.
adaptive value
The contribution of a trait to an individual’s ability to survive and reproduce.
adherent cup nest
A cup nest made of mud or saliva that relies on chemical forces to hold it to a vertical surface; built by many swifts, including the Edible-nest Swiftlets of Southeast Asia, whose nests are used in the Asian delicacy bird’s-nest soup.
In avian biology, the peaceful acquisition of a lone chick or chicks by a pair of adults other than the biological parents.
adrenal glands
Small yellow or orange endocrine glands at the cranial end of each kidney; they produce a variety of hormones (including adrenaline, steroids, and the sex hormones) that are involved with circulation, digestion, and reproduction.
advertising displays
Displays performed by one sex (usually the male) to attract a mate of the opposite sex; also called mate attraction displays.
aerodynamic power
For birds, the total power required to match the various forms of drag and thereby maintain flight.
aerodynamic valve
A vortex-like movement of air within the air tubes of each avian lung, at the junction between the mesobronchus and the first secondary bronchus; it prevents the backflow of air into the mesobronchus by forcing the incoming air along the mesobronchus and into the posterior air sacs.
African barbets
A family (Lybiidae, 42 species) of small, colorful, stocky African birds with large, sometimes serrated, beaks; they dig their nest cavities in trees, earthen banks, or termite nests.
Afrotropical region
Zoogeographic region including Madagascar, southern Arabia, and all of Africa south of the Sahara Desert. Sometimes called the Ethiopian Region.
A small feather that grows from the lower shaft of a contour feather and resembles the main feather but in miniature.
age structure
The relative proportions of individuals of different ages—usually noted for a given population.
age-specific fecundity
The average birth rate for females in a particular age group in a population.
age-specific survival rate
The proportion of individuals in a particular age group in a population that survive a particular interval of time—usually a year.
air sacs
Thin-walled, transparent sacs extending from the mesobronchi or the lungs to different regions of the body; they act as bellows to bring air into the body and store it until expiration. They are found only in birds.
air speed
A flying individual’s speed relative to the air through which it is moving. It does not include being carried along or slowed down by the wind, so it may or may not reflect a bird’s speed relative to the ground.
Any structure designed to help lift or control a flying object by using the air currents through which it moves. A typical airfoil, such as the wing of a bird or airplane, is rounded on top and curved inward below.
An individual that lacks the pigment melanin all over its body. An individual that lacks all types of pigments is called a complete albino.
Egg white; albumen is composed primarily of water and protein.
alimentary canal
The tube for the passage, digestion, and absorption of food; in most birds, it includes the esophagus, crop, two-part stomach, small intestine, ceca, and the large intestine. It is also called the gastrointestinal tract, digestive tract, or the gut.
In avian biology, the extra-embryonic membrane inside the egg that forms a sac into which the developing embryo shunts all metabolic wastes that cannot evaporate through the shell, such as uric acid crystals.
Allee effect
The response, shown in some species when population density falls below some threshold level, by which reproductive behavior and/or social structure become disrupted in various ways. In some circumstances, this effect may cause species to be unusually prone to extinction.
Alternate forms of genes. Most animals have two alleles for each trait
allometric scaling
Changes in the magnitude of a trait that are related to, but not directly proportional to, changes in magnitude of related traits.
Populations that occur in separate regions with no geographic overlap.
Mutual preening during which two birds preen each other, usually around the head and neck. In many species allopreening not only keeps the plumage clean and orderly, but also helps to establish social bonds between individuals.
alpha diversity
The count of the number of species present in a single habitat or location.
alpine tundra
Ecosystem found above the tree line on mountains; it consists of rugged, well-drained terrain interspersed with meadows with low-growing vegetation and a profusion of summer-blooming wildflowers. Very few birds breed in this harsh environment.
alternate plumage
In the Humphrey-Parkes system of nomenclature, alternate plumage is the plumage worn by an adult bird during the breeding season, and is produced by a partial molt before breeding. In the traditional system, this plumage was known as the nuptial plumage or the breeding plumage. If a bird does not molt before breeding, it continues to wear its basic plumage during breeding.
altitudinal migration
Seasonal migration up or down mountain slopes.
Describes young birds that hatch undeveloped and in many cases naked or with sparse down; such helpless young require complete parental care.
A group of two to six feathers projecting from the phalanx of the bird’s first finger (its thumb) at the bend of the wing. It reduces turbulence by allowing fine control of airflow over the wing.
American Ornithologists’ Union (AOU)
The largest organization of professional ornithologists in North America; it publishes the research journal The Auk and the Check-list of North American Birds (commonly called the AOU Check-list; see separate entry).
amino acids
A class of organic compounds that are the chief components of proteins
In avian biology, the extra-embryonic membrane inside the egg that becomes filled with fluid and surrounds the developing embryo, allowing it to move and stay moist, and preventing its various growing parts from sticking to or blocking one another.
A membranous chamber at the base of each of the three semicircular ducts in the inner ear; it contains sensory hair cells embedded in a gelatinous material and surrounded by endolymph, and it senses changes in the animal’s speed or direction in a particular plane of space. Information from all three ampullae, one in each plane of space, is combined by the brain to determine the animal’s motion and thus to aid balance.
angle of attack
In a flying bird, the angle between the main surface of the wing and the oncoming airstream.
anisodactyl feet
Foot arrangement in which the hallux points backward and the other three toes point forward. Found in most passerines.
annual survival rate
See survival rate.
Various species of tropical and subtropical trees in the genus Acacia that harbor ants inside their hollow thorns. The ants receive shelter and extra nutrition from special substances produced by the trees exclusively for the ants, and in turn keep away insects, mammals, and other herbivores that might feed on the trees by attacking them. The ants also prevent other vegetation from growing nearby by biting off shoots as they emerge from the ground.
Antarctic Convergence
Region of the oceans between about 50 and 60 degrees south latitude, where cold, north-flowing currents meet warmer, south-flowing currents, resulting in large-scale upwelling of nutrient-rich water. The nutrients support abundant plankton, which attract a great diversity and abundance of seabirds.
A Neotropical suboscine family (Thamnophilidae, 197 species) of small, insectivorous, forest birds; some species specialize in the technique of following columns of army ants to prey on the insects and other arthropods stirred up by the numerous moving ants.
The middle portion of the forelimb, consisting of the radius and ulna. The secondary feathers attach to the ulna.
Toward the front of an organism, using the earth as a frame of reference. With birds, technically used only within the eye and inner ear. But in practice, often used interchangeably with the term cranial.
anterior air sacs
General term referring to the air sacs nearest the bird’s front end—the cervical, clavicular, and anterior thoracic air sacs.
anterior chamber
Space within the eye between the iris and the cornea; it is filled with aqueous fluid, which nourishes the eye and removes wastes.
anterior lobe of the pituitary gland
Portion of the pituitary gland that receives instructions from the nervous system in the form of neurohormones from the hypothalamus. As a result, the anterior lobe secretes various hormones into the blood that may act directly on organs or on other endocrine glands (such as the gonads, adrenals, and thyroids); because of this central controlling role in the endocrine system, the anterior lobe is nicknamed “the master gland.”
The current geological age, marked by human activity having become a dominant influence on the earth’s climate and environments.
A blood protein able to attach to an infectious pathogen, thereby guiding other components of the adaptive immune system to attack that pathogen.
anting, active
Picking up an ant or other chemically potent object, such as a millipede, and deliberately rubbing it in the feathers—presumably to deter ectoparasites.
anting, passive
Positioning oneself among a swarm of ants, permitting them to run all over the body and to move in and out among the feathers, presumably to deter ectoparasites.
Singing interaction in which two individuals alternate their contributions. Often used to describe duets between a male and female bird.
antorbital fenestra
An opening on each side of the skull in front of the eye socket; found in all archosaurs.
Together with antthrushes, antpittas form the suboscine family Formicariidae (60 species). They are small, drab Neotropical birds with loud, ringing songs; many haunt the rain forest floor and may follow army ant swarms.
Together with antpittas, antthrushes form the suboscine family Formicariidae (60 species). They are small, drab Neotropical birds with loud, ringing songs; many haunt the rain forest floor and may follow army ant swarms.
The main artery exiting the heart; its branches distribute oxygenated blood to all parts of the body.
aortic arch
The curve of the aorta, just after it exits the left ventricle of the heart. In birds, the aorta curves to the bird’s right as it passes dorsal to the heart and toward the backbone, but in mammals, the aorta curves to the left.
See American Ornithologists’ Union.
AOU Check-list
Bird checklist produced by the Committee on Classification and Nomenclature of the American Ornithologists’ Union; it contains common and scientific names of all birds that occur in North America north of Mexico, or near North American coasts, including Hawaii, and is the generally accepted reference for common names of North American birds.
Shiny, broad sheets of connective tissue that bind muscle fibers together to form muscles.
Having bright, bold colors and patterns (often reds and oranges) that advertise to potential predators that an individual is bad tasting or poisonous. A few species that are perfectly palatable also have evolved these warning colors and patterns. The strategy of sporting aposematic coloration is called aposematism.
aposematic coloration
Often known as “warning colors,” conspicuous patterns or colors that signal to potential predators that an organism is potentially toxic or dangerous.
appeasement displays
Displays given to decrease the aggression of another individual; they usually consist of stereotyped postures that de-emphasize the performer’s weapons or size and expose vulnerable parts of its body. For example, a submissive bird may point its beak down or away, fold its wings, lower or turn away its head, point its tail down, or adopt some combination of these postures.
appendicular skeleton
Portion of the skeleton consisting of the sternum (breast bone), the pectoral girdle including the front limbs (wings), and the pelvic girdle including the hind limbs (legs).
apteria (singular, apterium)
Regions of bare or less-feathered skin between the feather tracts of birds.
aquatic birds
All birds with webbed feet that commonly swim, including the Anatidae; also, all deep-water waders belonging to the order Ciconiiformes, such as herons and storks. Also called water birds.
aqueous fluid
A cell-free fluid, similar in structure to blood plasma; it fills the anterior and posterior chambers of the eye (between the lens and the cornea), providing nourishment and waste removal.
The middle of the three vascularized membranes, called meninges, surrounding the brain and spinal cord. The meninges provide sustenance and waste removal for the cells of the brain and spinal cord, which are not served by the circulatory system.
Living in trees.
arboreal theory (of the origin of avian flight)
Theory suggesting that flight originated in small, arboreal (tree-dwelling), reptile-like birds that jumped or glided among tree branches. Proponents suggest that feathers first evolved to keep the animals warmer in the cooler arboreal environment, and then were used to extend jumps or glides. First proposed in 1880 by O. C. Marsh. Also called the trees-down theory.
A feathered reptile from 150-million-year-old Jurassic limestone deposits. Because it had a mosaic of bird and reptile characteristics, its relationship to modern birds and other reptiles has been highly controversial since its discovery in the early 1860s.
All reptiles in the Archosauromorpha, the large group of diapsid reptiles that includes all thecodonts and their descendants (including birds and crocodiles), but does not include lizards, snakes, or turtles. All archosaurs have an opening on each side of the skull, in front of the eye socket, called the antorbital fenestra.
arctic tundra
Ecosystem found around the world in a belt extending north from the limit of trees; it consists of swampy flatland covered with low-growing vegetation, interrupted by countless shallow lakes. Very few birds live in the arctic tundra year round, but many migrants breed there, taking advantage of the long hours of daylight and numerous spring insects for feeding their young.
arctic tundra/coniferous forest ecotone
Transitional zone between the arctic tundra and coniferous forest ecosystems, sometimes called the northern timberline. It consists of fingers of open, stunted spruce forest extending north between fingers of low, shrubby tundra extending south.
area sensitivity
Refers to bird species that are found only in habitat patches larger than some critical size.
area-sensitive species
Species for which probability of occurrence in a habitat type increases with the size of the habitat patch, hence are usually missing from the smallest patches.
army ants
A subfamily of primarily tropical ants that are highly social and nomadic
Small blood vessels branching from arteries. They carry blood from the arteries to the capillaries.
A vessel conducting blood away from the heart. All arteries except the pulmonary artery carry oxygen-rich blood.
articular bone
Bone on the upper surface of each side of the lower jaw of many vertebrates, near the caudal end; in birds, it links with the quadrate bone of the upper jaw, forming the joint between the jaws.
artificial selection
The evolutionary process analogous to natural selection that results when humans are the selective agent that determines which individuals are allowed to reproduce; often this occurs when humans selectively breed individuals for desirable traits.
arytenoid cartilages
Two cartilages of the larynx; they stiffen and hold the shape of the fleshy folds surrounding the glottis—the opening of the larynx.
Asian barbets
A family (Megalaimidae, 26 species) of chunky birds slightly smaller than a Belted Kingfisher, with thick bills and gaudy, clashing colors. They are endemic to the Oriental zoogeographic region.
Members (with false sunbirds) of the family Philepittidae, endemic to Madagascar. These two species of suboscine passerines feed on fruits.
aspect ratio
The ratio of an object’s length to its width. It is used to refer to the shape of a bird’s wing.
assembly rules
The hypothesis that competition among species creates possible and impossible combinations of particular species within biological communities.
asynchronous hatching
Pattern of hatching in which the eggs of a single clutch hatch over a period of several days, resulting in a brood of young of different ages. This pattern occurs when incubation begins at the time the first egg is laid. Because eggs are laid one per day, at one- to two-day intervals, the embryos of the earliest-laid eggs have already started to develop by the time the later eggs are laid, and they hatch sooner.
The first cervical vertebra; in birds, it articulates with the single occipital condyle on the base of the skull. (Mammals have two occipital condyles.)
atria (singular, atrium)
The two thin-walled, anterior chambers of the heart; they receive blood returning to the heart from the lungs (left atrium) or body (right atrium).
atrioventricular valve
The valve between each atrium and its corresponding ventricle; it prevents the backflow of blood into the atrium as the ventricle contracts.
To shrivel or die back; may be pathological or part of the normal course of development.
attentive periods
In avian biology, periods spent on the nest during incubation.
auditory nerve
See vestibulocochlear nerve.
auditory tube
Air-filled tube leading from the middle ear to the throat; it helps to equalize the air pressure on the two sides of the eardrum. In birds, the right and left tubes join and enter the caudal roof of the mouth through just one opening. In mammals, the right and left tubes enter the mouth separately. Also called eustachian tube.
auricular feathers
A patch of feathers covering the external ear opening. Their open texture protects the ear from debris and wind noise, yet helps to channel sounds into the ear.
austral migration
Refers to seasonal migration in the southern hemisphere, in which birds typically move north in Fall and south in Spring.
Australasian region
One of the major zoogeographic regions of the world, stretching from a line termed “Wallace’s Line” east of the islands of Timor and Sulawesi in Indonesia, southwest to New Zealand, and including New Guinea, Australia, Hawaii, and other islands of the mid-Pacific Ocean. Sometimes called the Australian region.
Australasian robins
A family (Eopsaltriidae, 44 species) of songbirds endemic to the Australasian region. Reminiscent of both New and Old World robins in coloration, they actually are more like flycatchers, although they usually snatch food from the ground.
autonomic nervous system
A set of nerves considered as a group because of their similar functions. Acting primarily unconsciously, they innervate the smooth muscle of the viscera, glands, and blood vessels, thus controlling the automatic function of the internal organs. They are under direct chemical control from substances circulating in the blood.
The set of bird species living in a region.
axial skeleton
The portion of the skeleton consisting of the skull, hyoid apparatus, and the vertebral column of the neck, trunk, and tail.
A cluster of feathers in the bird’s “armpit”; they are recognizably longer than those lining the wing.
The second cervical vertebra.
The cable-like, impulse-conducting, main axis of a neuron.


B cells
Special white blood cells found in the lymphatic tissues; they are important in the immune response because they produce antibodies. B cells are produced by the cloacal bursa and are also important in understanding the development of AIDS in humans.
A diverse family (Timaliidae, 267 species) of gregarious, insectivorous birds, many of which have complex social systems and breed cooperatively. They are found in the Afrotropical, Oriental, and Australasian regions.
In birds, the dorsal side of the body, between the neck and the rump.
barbicels (or hooklets)
Tiny hooks on the barbules that allow them to attach to adjacent barbules, thereby forming the flat surface of a feather vane.
The parallel branches extending from each side of the rachis of the feather shaft; collectively they form the vanes.
Branchlets coming off both sides of the barbs of a feather, at right angles to the barbs and in the same plane. Adjacent barbules hook together, holding the vane intact.
basal archosaurs
See thecodonts.
basal metabolism (metabloic rate)
The number of calories an organism uses when completely at rest, which indicates the amount of energy needed to maintain minimal body functions.
basic plumage
In the Humphrey-Parkes system of nomenclature, the plumage worn by an adult bird for the longest time each year; it usually is produced by a complete molt. In the traditional system, this plumage was known as the nonbreeding or winter plumage. If a bird does not molt before breeding, it continues to wear its basic plumage during breeding.
basilar papilla
The lower membrane of the cochlear duct in the inner ear of birds; it is coated with a layer of sensory hair cells. Sound waves set the basilar papilla into motion, causing the hair cells to push against the tectorial membrane, triggering nerve impulses in the hair cells that are sent to the brain in the process of sound perception. In mammals, the corresponding structure is called the “basilar membrane.”
See Breeding Bird Survey.
A bird’s upper and lower jaws, including the external covering; also called the bill.
An Old World family (Meropidae, 26 species) of brightly colored birds with long, slender beaks.They catch stinging insects in a manner similar to that of flycatchers, and then beat them to remove the stingers before eating them.
behavioral isolation
A form of reproductive isolation involving incompatibilities in behavior that prevent the interbreeding of two populations. These can include behaviors involved in recognizing members of the same species, specialized courtship displays, or any other behaviors necessary for successful mating.
In birds, part of the lower (ventral) surface of the body between the breast and the vent.
bend of the wing
The prominent angle at the wrist, where the bird’s wing bends noticeably.
benefits of philopatry hypotheses
A set of possible explanations for why certain individual birds might forego their own breeding in a particular breeding season and act as helpers at the nest of other breeding pairs (usually their parents or other close relatives); the benefits of philopatry hypotheses focus on the possible benefits to young adults of remaining with their parents. Examples of these hypotheses include (1) the survival of young adults may be improved when they remain in a group, (2) by helping, young adults may improve the survival of close relatives (and thus increase their own indirect fitness), and (3) by staying with their parents, young adults may increase their own chance of acquiring a superior territory, either by monitoring vacancies in neighboring sites or by inheriting their natal territory.
Bergmann’s rule
Rule describing the pattern of body sizes found within most bird and mammal species, in that individuals living in colder regions tend to be larger than those living in warmer areas.
Bernoulli’s law
Physical law stating that static and dynamic pressure must always add up to a constant. Because the airflow over a moving airfoil (such as a bird’s wing) is faster above than below, the dynamic pressure is higher and thus the static pressure is lower. Because the static pressure below the wing is higher than that above, lift is created and the bird can remain aloft.
beta diversity
The accumulation of species that results as a sampled area includes greater number of habitats, each with some species found only in that habitat.
Describes an egg that is slightly longer than subelliptical; also called fusiform or long subelliptical.
Substance produced by the liver that emulsifies fats to facilitate their digestion. In birds with no gall bladder, bile is released directly into the small intestine through the hepatoenteric ducts; in birds with a gall bladder (and mammals), bile is stored in the gall bladder and released through the bile ducts.
A bird’s upper and lower jaws, including the external covering; also called the beak.
bill tip organ
An aggregation of sensory cells at the tip of both the upper and lower beak, best developed in ducks, geese, sandpipers, and snipe; it is thought to sense tactile stimuli during feeding.
A maintenance behavior in which a bird swipes its bill sideways on tree branches, the ground, or other surfaces, especially after eating messy foods such as oily insects or suet.
binocular vision
A type of vision that produces three-dimensional images, in contrast to monocular vision, which produces flat images. Binocular vision results when the eyes are positioned toward the front of the head, so that objects are detected by both eyes simultaneously.
binomial name
A naming system in biology in which each species is assigned a unique name consisting of two parts, the name of the genus and the name of the species, as in the scientific name of the House Sparrow, Passer domesticus.
binomial nomenclature
The currently accepted system of naming organisms, devised by Linnaeus, in which each species is designated by two words
The increase in the concentration of toxic or other foreign substances in organisms’ bodies as a result of taking up the substances from the environment (through plant roots, or in ingested food or water) at a rate higher than that at which they are excreted from the body. Many toxic substances that do not occur naturally, such as DDT and PCBs, are readily deposited in body tissues but excreted at very slow rates, because organisms have not evolved mechanisms to metabolize them effectively.
The step-wise increase, found at each higher level of the food chain, in the concentration of certain chemicals in the bodies of organisms. Chemicals that usually bioconcentrate in the food chain are those toxins, such as DDT, that tend to accumulate (see bioaccumulation) in organisms because they are taken up from the environment faster than they are excreted. Also called biological magnification.
The great wealth of living organisms that occur on earth.
biodiversity hotspot
A location or region with an exceptionally high diversity of species.
biogeographic realms
A traditional classification of the earth’s terrestrial regions that groups them by their broadly similar faunas, floras, and evolutionary histories.
The study of the distribution patterns of living things.
biological indicators
See indicator species.
biological magnification
See bioconcentration.
biological species concept
A classic species concept that is still widely used in Ornithology; it places the greatest emphasis on whether or not the members of two populations retain the ability to interbreed.
The total mass of all the living organisms in a particular population, community, or area at a given point in time.
An ecological community of organisms that occurs across a broad geographic region defined by its location, biogeographic history, and climate.
biotic factors
Features of the environment that result from living organisms, such as the presence or absence of competitors, predators, prey, etc.
Walking (or running) on two legs.
bird community
See community.
A family (Paradisaeidae, 46 species) of forest-dwelling songbirds found primarily in New Guinea; they are famous for the spectacular, colorful plumages and displays of the males.
birth rate
The number of young born to an individual or set of individuals, or born into a population or species, in a given period of time (often a year). Usually (especially for humans) expressed as the number of births per 1,000 individuals per year. Also called fecundity.
In avian biology, the flattened disc of dividing cells that lies on the upper surface of the yolk and is the first stage in the development of the embryo.
A structure used to conceal a person so that he or she may observe birds or other wildlife; known as a “hide” in Great Britain.
blind spot
Site on the retina where the optic nerve penetrates the retina and leaves the eye; because no rods or cones are present at this spot to capture incoming light, an object whose image falls on the spot is not perceived.
A method of estimating the size of large flocks of birds by counting the birds in a block of typical density, beginning at the trailing end of the flock (so that birds are not flying into the area you are counting), and then visually superimposing the block onto the rest of the flock to see how many times it will fit.
blood plasma
See plasma.
blood vascular system
Alternate name for the circulatory system.
blood/brain barrier
The specialized arrangement of capillaries in the brain that prevents blood from reaching the nerve cells, thus protecting the brain from potentially toxic substances that circulate in the blood. Capillaries penetrate the meninges surrounding the brain, but do not reach the actual neurons of the brain.
body downs
The down feathers of adult birds, found under the contour feathers. Body downs are most common in water birds and hawks, as they provide extra insulation.
Tissue composed of living cells in a mineralized matrix; the bones provide support for the body and attachment sites for the muscles.
bony labyrinth
The bony, outer system of fluid-filled canals (containing perilymph) that make up the inner ear. It forms the cochlea, semicircular canals, and vestibule, and encloses the membranous labyrinth.
booming sacs
Brightly colored outpocketings of the esophagus of some North American grouse that appear at the sides of the neck; they fill with air and act as resonators to produce loud sounds during displays.
booted podotheca
A smooth podotheca, divided into long, continuous, nonoverlapping scales.
boreal forest
Coniferous forest ecosystem dominated by spruce and fir trees and found around the world, generally in a belt north of temperate zone deciduous forests and south of the arctic tundra. Many birds migrate to the boreal forests to breed, taking advantage of the long daylight hours and abundant insects for feeding their young. Some bird species are year-round residents. Also called taiga.
bottom-up effect
In ecology, a situation when the abundance of a consumer or predator—or important parts of the ecosystem overall—are determined primarily by the degree of primary production at the bottom of the food chain.
A flight pattern in which a bird alternates flapping (during which it rises slightly) with glides on closed wings (during which it descends slightly).
A complex mating structure built by male bowerbirds—members of the passerine family Ptilonorhynchidae. See bowerbirds.
A passerine (oscine) family (Ptilonorhynchidae, 20 species) of New Guinea and Australia whose polygynous males attract females by building and decorating remarkably complex “bowers” out of twigs and other objects.
brachial plexus
Plexus along the spinal cord of birds at the level of the wing; it is associated with a cervical enlargement of the spinal cord.
The upper (proximal) portion of the forelimb (wing); it contains the humerus.
brain stem
See medulla oblongata.
The lines on a phylogeny that trace the pathway of evolutionary lineages through time.
In birds, part of the lower (ventral) surface of the body, between the throat and belly.
Breeding Bird Survey (BBS)
A count of the breeding birds of North America conducted each summer since 1967 and coordinated by the U. S. Fish and Wildlife Service. Observers count birds seen or heard during three-minute periods at half-mile (0.8-km) intervals along a 25-mile (40-km) stretch of road. Because the same routes and stops are sampled each year, BBS data can be used to track population trends, and the results can be correlated with habitat.
breeding dispersal
The movement of an individual bird in successive years among different breeding locations.
breeding plumage
See alternate plumage.
breeding season
The period of time during the year when a particular species may breed.
Highly specialized contour feathers in which the rachis is stiffened and lacks barbs along its outermost parts.
A family (Eurylaimidae, 15 species) of stocky, brightly colored, suboscine birds found throughout forests and scrublands of the Old World tropics, particularly in the Oriental region. Broadbills use their wide, flat, colorful bills to snatch up large insects.
broad-front migration
A pattern in which the migrating individuals of a species are spread across a wide area rather than concentrated into narrower flyways.
bronchi (singular, bronchus)
The two major air tubes branching off the lower end of the trachea; one goes into each lung.
brood parasite
A bird that lays eggs in the nests of other species, leaving the resulting young to be raised entirely by the host parents. Some species parasitize others only occasionally; others, called obligate brood parasites, never build their own nests, and lay all their eggs in other species’ nests.
brood parasitism
Any situation in which a female bird lays its eggs in the nest of another individual of the same or different species.
brood patch
A patch of skin on the breast and belly of birds that has lost feathers and become swollen through both the retention of large amounts of water in the tissues and the expansion of blood vessels feeding the skin. It develops a few days before egg laying in most individual birds (either male or female) that incubate their eggs by sitting on them, and increases the efficiency of heat transfer to the eggs. One large patch or several smaller ones may develop, depending on the species.
brood reduction
Practice carried out by some parent birds whose young hatch asynchronously and thus vary in size, in which the parents first feed the most vigorously begging offspring until it can swallow no more, and then move on to another. Thus, in years with low food supplies only the largest and strongest young will survive, but in years with abundant food smaller young will survive as well. Brood reduction ensures that at least some young will survive in years when food is scarce.
Sitting on hatched young, or sheltering them under the wings, primarily to keep them warm, but also to protect them from sun, rain, or predators. Occurs either in the nest, or outside the nest in those species whose young leave the nest shortly after hatching.
Brown-headed Cowbird
See cowbird.
burrow nest
A cavity nest in the ground that is longer than it is high; usually accessed by an entrance tunnel.
bursa of Fabricius
Former name for the cloacal bursa.
A family (Otididae, 25 species) of large, heavy-bodied, flat-headed birds with long legs and necks. These ground-dwelling birds are strong runners, and frequent open areas in the Oriental, Australasian, and southern Palearctic regions, although most species live in Africa.


The behavior of storing food in an often hidden location (a “cache”) for later retrieval and consumption.
In birds, side-pouches that open into the large intestine or rectum.
The hollow lower portion of the feather shaft, part of which lies beneath the skin; it has no vanes.
An avian vocalization that is shorter and simpler than a typical song.
call notes
Bird sounds that are generally shorter and simpler than songs. Many seem to convey a specific message, such as begging calls (hunger), alarm calls (danger), and contact calls (the caller’s location).
See chorioallantoic membrane.
The curvature of the wing as seen through a cross-section; highly cambered wings are more arched and therefore the upper and lower surfaces are less symmetrical.
Coloration of an organism or structure serving to conceal it from predators, other enemies, or prey.
The upper, continuous level of vegetation in a forest; it contains the branches of tall, mature trees and the epiphytes that grow on these branches. In dense forests, the canopy receives most of the sunlight and thus has the greatest productivity. The tallest trees that stick above the main canopy, somewhat like “lollipops,” are not considered part of the canopy; they are called the emergent layer.
The smallest blood vessels. Within all body tissues except the epidermis and those in the central nervous system, capillaries form networks called capillary beds, which connect the arterioles and venules. The thin walls of the capillaries allow materials to be exchanged between the blood and the body cells—the body cells absorb oxygen from the blood, and the blood takes up wastes from the body cells.
capillary bed
See capillaries.
cardiac muscle
A special type of smooth muscle that forms the bulk of the heart. Cardiac muscle can contract without being stimulated by the nervous system.
A midventral ridge of bone that projects outward from the sternum and provides an attachment site for the pectoral flight muscles. Synonymous with keel.
Birds that have a keel (carina) on their sternum; includes all flying birds and many smaller flightless ones.
Pigments producing bright yellow, orange, or red colors. They are synthesized only by plants, so birds must obtain them via their diets.
carotid artery
Main artery supplying the head and neck region; among birds it is highly variable, with different species having none, one, two equal in size, or one large and one small.
The bones of the wrist. In birds, they are fused and reduced to just two bones—the radiale and ulnare.
The largest bone of the manus of birds, formed by the fusion of some of the carpals (wrist bones) with the metacarpals (palm bones).
The flesh of dead animals.
carrying capacity
The maximum population size or density that a particular area can support over the long term, without any degradation in the quality of the area or its resources.
A tissue with living cells embedded in a nonmineralized matrix; cartilage is found in the flexible joints and is capable of growth or resorption as well as transformation into bone.
Enlargements on the top of the bill or the front of the head, usually involving the underlying bone. Found in cassowaries and hornbills, among others.
A family (Casuariidae, 3 species) of large, flightless ratites inhabiting New Guinea and Australia. They have a distinctive bony casque on top of their head.
See pellet.
Toward the tail or posterior part of the body.
caudal mesenteric vein
Vein that in birds brings blood from the lower portion of the digestive tract to the venous ring within the kidney; it is part of the avian renal portal system.
caudal vena cava
Single large vein that gathers blood from veins coming from the legs, tail, kidneys, and caudal regions of the body and delivers the blood to the right atrium of the heart.
caudal vertebrae
The vertebrae of the tail. In birds, the anterior caudal vertebrae fuse with the sacral and lumbar vertebrae and some of the thoracic vertebrae to form the synsacrum; the next four to nine vertebrae articulate freely; and the posterior ones fuse to form the pygostyle (tail bone).
caval veins
General term for the right and left cranial vena cavae and the caudal vena cava; these large veins gather deoxygenated blood from the body and return it to the right atrium of the heart.
cavity adopters
Birds that nest in cavities but do not excavate their own, instead obtaining cavities that were created by physical forces (such as decay or erosion) or by other species. Also called secondary cavity nesters.
cavity nest
A nest located in an empty space within a larger substrate, such as the cavities excavated by woodpeckers or the nest-boxes provided by humans.
See Christmas Bird Count.
Membrane-enclosed unit capable of metabolism and reproduction; the basic structural and functional unit of life.
cell body
The part of a neuron containing the nucleus and surrounding cytoplasm; also called a soma.
center of origin (of a species)
The geographic area where a species evolved.
central fovea
Area in the central part of the retina of most birds and mammals where the cones are most concentrated and the neural layer (the nerves from the rods and cones, which overlay the rods and cones and block some light) is the thinnest, and thus vision is the sharpest. The central foveae provide sharp monocular views of the areas to the sides of the bird.
central nervous system (CNS)
The brain and spinal cord.
The main body, or central axis, of a vertebra; the anterior end of the centrum connects to the preceding vertebra, and the posterior end connects to the following vertebra.
A leathery band of skin covering the base of the bill, into which the nostrils open; presumably the cere protects the nostrils. Present only in certain birds, such as hawks, pigeons, and some parrots.
A large, deeply folded structure on the dorsal surface of the hindbrain, attached to the brain stem by two pairs of stout neural tracts; it controls muscular coordination and plays an important role in balance, posture, and proprioception. The cerebellum of birds is particularly large, due to the demands of flight.
cerebral hemispheres
The two large, smooth lobes on the dorsal anterior region of the forebrain; together they form the cerebrum, which coordinates and controls complex behaviors, including memory and learning. In mammals, these lobes have folds and grooves.
See cerebral hemispheres.
cervical air sacs
A pair of air sacs—one sac on each side of the body—located in the neck region of birds; one sac usually extends from each lung, but sometimes a series of cervical sacs are located along the neck, as in geese.
cervical enlargement
A swelling along the spinal cord at the level of the wings, associated with the brachial plexus.
cervical vertebrae
The vertebrae of the neck region; birds have uniquely shaped centrum ends of their cervical vertebrae (see heterocoelous centrum ends).
In avian biology, the gelatinous, usually milky white, stringy coils of albumen (egg white) that surround and protect the egg yolk, and are visible at either end of the yolk as twisted cords. The chalazae attach to the far ends of the eggshell and form a suspension system for the yolk that allows it to rotate throughout embryonic development.
The process by which humans deepen and straighten natural streams, converting them into water-filled ditches. Theoretically channelization controls flooding along the stream, but it destroys the stream ecosystem and often increases flooding downstream.
North American ecosystem found on the low hillsides of southwestern California; it consists of dense stands of broad-leaved evergreen shrubs, dominated by chamise and manzanita. This ecosystem is dry, with long hot summers with frequent fires; rain falls only in winter. A moderate number of birds breed in the chaparral.
In avian systematics, any heritable trait that can be compared among different groups of birds.
character displacement
Situations in which two competing species are more dissimilar in places where they both occur, compared to locations where only one species is present.
A printed list of the birds found in a particular area.
See malar region.
A small area under the lower beak of birds.
A single slit in the roof of the mouth, running in an anterior-posterior direction, through which the two nasal cavities open to the mouth.
chorioallantoic membrane (CAM)
Membrane covering the entire inner surface of the avian eggshell, inside the inner shell membrane; it develops part-way through embryonic development from a fusion of two extra-embryonic membranes, the chorion and allantois. The CAM is richly invested with blood capillaries, and together with the pores in the eggshell, it allows the embryo to carry out gas exchange as it receives oxygen from outside the egg and expels carbon dioxide.
In avian biology, the extra-embryonic membrane inside the egg that surrounds the entire avian embryo and the other three extra-embryonic membranes (the allantois, amnion, and yolk sac). It is homologous (evolutionarily related) to the mammalian membrane, also called the chorion, which forms much of the placenta in most mammals.
The middle layer of the three main layers of the eye; it lies just inside the sclera. It is pigmented and forms the iris and ciliary processes.
Christmas Bird Count (CBC)
A count of the wintering birds of North America conducted each year since 1900 and coordinated by the National Audubon Society. Observers count as many individual birds as possible within one of the circular count areas 15 miles (24 km) in diameter that are scattered across North America and beyond. Observers record their time spent and distance covered, so the numbers of birds seen can be adjusted for observer effort. The data can be used to track winter bird distribution and abundance, as well as long-term population trends.
Long strands of DNA found in the nuclei of most cells. Each section of the chromosome (a specific sequence of nucleotides) that codes for a specific protein is called a gene.
ciliary muscles
Muscles that, in birds, attach to the ciliary processes, which attach to the lens of the eye. When the ciliary muscles contract, they move the ciliary processes, which squeeze the lens and make it become more round. In mammals, ciliary muscle contraction relaxes the lens, allowing it to become round by elastic rebound.
ciliary processes
Structures of the choroid of the eye that attach to the lens and hold it in place. Ciliary muscles move the ciliary processes, which in turn move the lens, changing its shape. Only in birds do the processes attach directly to the lens.
circadian rhythms
Daily cycles of behavioral and physiological events exhibited by organisms; they are regulated by an internal biological clock and persist even when organisms are kept under constant environmental conditions. Examples include daily patterns of activity, body temperature, or nectar production.
circannual rhythms
Cycles of behavior, growth, or other physiological activities that occur on approximately a yearly basis; like circadian rhythms, they are regulated by an internal biological clock and persist even when organisms are kept under constant environmental conditions. Examples include yearly patterns of migration, shedding and regrowth of antlers, and hibernation.
circumpolar constellations
The stars near to and surrounding the North Star (Polaris).
A family (Cisticolidae, 117 species) of small, drab, insectivorous warbler-like birds of open, grassy areas of the Old World, primarily Africa.
See Convention on International Trade in Endangered Species.
citizen science
Scientific endeavors in which members of the public are core participants in collecting data and/or interpreting results.
A noun that is a synonym for monophyletic group, referring therefore to any group of organisms that includes the most recent common ancestor of that group and all of its past and present descendants.
Level of classification of organisms above “order” and below “phylum”; similar orders are placed within the same class. All birds are in the class Aves.
In an evolutionary context, the process by which scientists name organisms and assign them into larger group based on their evolutionary relatedness.
The collar bone; in nearly all birds, the left and right clavicles fuse with a small interclavicle bone to form the V-shaped furcula (wishbone).
clavicular air sac
A single, median air sac between the clavicles and surrounding the bifurcation of the trachea of birds.
claw arc
The angle between the tip and base of a claw, considering the claw as a section (arc) of a circle; used by researchers to compare the curvatures of different claws. It is significant because claw curvature is related to a bird’s habits
clay lick
Clay banks rich in minerals, such as calcium, that are visited by frugivorous or seed-eating birds, such as parrots and macaws, who eat the clay to obtain minerals that are otherwise lacking in their diet. Clay licks are one type of mineral lick.
Clean Water Act
Federal law passed in the United States in 1972 that attempted to improve the quality of surface waters by controlling pollution and requiring sewage treatment. Section 404 (see separate entry) contains a set of protections for wetlands.
climax community
The association of plants and animals (and other organisms) that can perpetuate itself in a given area in the absence of large-scale climatic change, disease, or human disturbance. The climax community is the final stage in the process of ecological succession in any given area.
A gradual change in certain characteristics of individuals of the same species, which is evident in a geographic progression from one population to the next.
Common opening at the lower end of the avian digestive tract for the digestive, excretory, and reproductive systems; it receives feces from the large intestine, uric acid from the kidneys, and eggs or sperm from the gonads, and releases these materials through the vent.
cloacal bursa
A lymphoid organ that opens into the roof of the cloaca in young birds and atrophies in later life; it produces special white blood cells called B cells, which are important in immune function and are of interest to researchers for their role in the development of AIDS in humans. Previously known as the bursa of Fabricius.
cloacal phallus
The copulatory organ of male ratites and waterfowl; it is an elongate, spiral, ridged structure that erects by lymphatic pressure during copulation. Sperm travel along its surface to reach the cloaca of the female.
cloacal protuberance
A swelling at the caudal end of the deferent ducts, visible externally in hand-held birds with the feathers parted; it is present only in breeding male passerines, and is used by researchers to determine sex and breeding condition. The swelling is caused by the enlargement of structures in the terminal regions of each deferent duct during the breeding season, and may help to keep sperm slightly cooler than the body’s core temperature (as does the scrotum of mammals).
A complete set of eggs; those laid in an uninterrupted series, for a single nesting, by one female.
clutch size
The number of eggs in a given clutch.
Central nervous system; the brain and spinal cord.
Of the coast; coastal bird species primarily occupy the shallower waters around oceanic islands or above the continental shelf, feeding mainly on fish, crustacea, and mollusks, which they find on or near beaches and other shorelines. They visit land frequently, during both the breeding and the nonbreeding season.
Elongate, bony, structure of the inner ear that is concerned with hearing; it is part of the bony labyrinth and thus is filled with perilymph. It contains the vestibular canal and tympanic canal connected at one end and filled with perilymph, and the cochlear duct, which lies between them. In mammals, but not in birds, the cochlea is coiled like a snail’s shell.
cochlear duct
Membranous canal inside the cochlea, bounded above by the tectorial membrane and below by a membrane that in birds is called the basilar papilla. The cochlear duct is part of the membranous labyrinth and thus is filled with endolymph.
cochlear window
A soft spot at the end of the bony cochlea farthest from the vestibular window; it acts both as a pressure-release valve and as a damper for the waves in the cochlea. Formerly called the round window.
An evolutionary interaction between two or more species in which one evolves an adaptation that affects another, and then the other evolves an adaptation in response, and then the first evolves another adaptation in response to the response, and so on. Coevolution sometimes results in a kind of “battle” or “arms race” between two species. Notable examples of coevolution include interactions between predators and prey, plants and pollinators, and brood parasites and hosts.
A group of individuals born during the same time period (often a year).
cold front
The interface between a mass of cold air and the warm air mass it is overtaking; the dense, cold air tends to wedge under the warm air, forcing the warm air up and cooling it abruptly, forming precipitation that is often accompanied by strong winds and lightning.
See ectothermic.
colic ceca (singular, cecum)
Pouches extending from the junction between the small and large intestines that hold partly digested food long enough for bacterial action to further break it down; digested material is released to the large intestine, where any released nutrients are absorbed. Birds may have none, one, or one or two pairs of colic ceca, and the size is highly variable among species. Also spelled caeca or caecum.
See mousebirds.
color phases
Polymorphisms in which the morphs differ in color; for example, the red and gray phases of Eastern Screech-Owls and Ruffed Grouse, and the blue and white phases of Snow Geese.
Small, thin bone extending across the middle ear of birds, attached at one end to the inner surface of the eardrum and at the other end to the vestibular window of the inner ear; it transmits sound waves from the eardrum to the fluid-filled cochlea.
Fleshy, erect structure positioned longitudinally on top of the head of a bird, often with a serrated margin (like a hair comb), as in domestic chickens.
Describes a relationship between two species or individuals in which one benefits and the other neither benefits nor is harmed. For example, when Cattle Egrets follow large herbivorous mammals to eat the insects they stir up as they move, the egrets benefit but the mammals appear unaffected in any way.
An ecological relationship involving two species, in which one species benefits without influencing the other species either positively or negatively.
communal roost
A group of birds gathered to spend the night together, sleeping; may consist of just one species or a number of different species. Birds that form particularly large and noisy communal roosts include vultures, ravens, crows, starlings, herons, egrets, ibis, grackles, blackbirds, cowbirds, robins, and the extinct Passenger Pigeon.
All the populations of species living and interacting with one another in the same place. Communities also can be defined as containing only certain types of species in one location, such as a forest bird community or a stream insect community.
competitive exclusion principal
Rule stating that no two species can occupy exactly the same ecological niche in a community; if they did, then eventually one would outcompete the other and cause it to go extinct.
complete albino
An individual lacking all types of pigments in the plumage, eyes, and skin.
complete molt
A type of molt in which the entire feather coat is replaced.
Two thin, scroll-like structures (one median, one anterior) extending from the lateral wall of each nasal cavity; they are covered with a mucus-secreting membrane that contains the nerve endings of the olfactory nerves, which sense odors. The mucus traps dust, and blood vessels in the membrane warm the inhaled air.
One of the two kinds of light-sensitive cells lining the retina of the eye; they are responsible for visual acuity (due to their tight packing) and for sensing color information (via four or five pigments and specialized oil droplets in birds), but are not very sensitive to low light levels. When light energy stimulates a cone cell, it sends a nerve impulse to the brain via the optic tract.
See pyriform.
coniferous forest
Ecosystem of colder parts of the temperate zone, where there is sufficient moisture to support a forest; cone-bearing trees, especially spruce and fir, dominate the vegetation. North America contains three main types of coniferous forests
coniferous forest/deciduous forest ecotone
Transitional zone between the coniferous forest and deciduous forest ecosystems; it has a mixture of the two types of trees and, in North America, has more species of breeding birds than any other region.
conservation biology
An applied science that combines information gained through the biological fields of ecology, population biology, animal behavior, and genetics to attempt to reverse the widespread declines and extinctions of species occurring throughout the world today.
conservation easement
A legal agreement in which a landowner permanently limits uses of their land in order to protect its conservation value.
conservation plan
See habitat conservation plan.
Conservation Reserve Program (CRP)
Part of the 1985 Food Security Act that authorized the USDA to lease millions of areas of marginal croplands from farmers each year, paying them to keep the land in perennial vegetation to reduce soil erosion and crop surpluses, and to restore wildlife habitat. The program has helped prairie songbirds by providing millions of acres of grassland habitat.
conservation-reliant species
Species that will require active, ongoing, and perhaps permanent conservation measures to avoid extinction.
Members of the same species.
contact call
A sound produced by a bird that appears to inform a nearby bird (usually a family member) of the caller’s location. Often uttered by a mated male and female as they forage relatively close together.
contour feathers
Feathers that make up the exterior surface of a bird, including the wings and tail; they streamline and shape the bird, and usually have well-developed barbules and hooklets.
Convention on International Trade in Endangered Species (CITES)
International agreement to which 150 nations voluntarily subscribe that binds participating parties to monitor, regulate, or prohibit the import and export of species that the group has deemed worthy of global protection. The species are listed in three Appendices
convergent evolution
The process by which organisms evolve similar forms, behaviors, or ecological characteristics not because they are related, but to meet similar environmental challenges.
cooperative breeding
Breeding system in which adults other than the breeding pair help the breeding pair to rear their offspring. In birds, if the helpers assist with incubation or care of nestlings, they are called “helpers at the nest” (see separate entry).
cooperative foraging
A technique in which a group of individuals work together to obtain prey. For example, American White Pelicans may swim in a line or semicircle and beat their wings to drive schools of fish into shallow water. Other pelicans, cormorants, and mergansers, among others, forage cooperatively some of the time. Also called social foraging.
copulation-solicitation display
A stereotyped posture adopted by female birds that indicates their readiness to copulate.
Strong, stout, paired bones of a bird’s pectoral girdle; they are not present in mammals. During flight the coracoids function as a powerful brace holding the shoulder joint, and thus the wing, away from the body while the pectoral muscles pull on the wing in the opposite direction.
The two members of the passerine (oscine) family Corcoracidae—the blackbird-like White-winged Chough and the smaller, seed-eating Apostlebird. These large, Australian cooperative breeders range over agricultural fields in huge flocks when not breeding.
core area
The portion of a large, continuous habitat, such as a forest, that is far from the edges and thus is suitable to host species that would be adversely affected if forced to live near the edges.
The transparent anterior surface of the sclera; it allows light to enter the eye.
coronary arteries
Arteries (usually two) arising from the first part of the aorta; they carry blood that nourishes and supplies oxygen to the heart muscle itself.
coronary veins
Veins that carry deoxygenated blood and wastes from the muscle tissue forming the heart back into the right atrium of the heart.
An alternate name for blood cells.
In conservation biology, long, narrow areas of wildlife habitat that connect larger areas, thus allowing individuals to move between the larger areas.
countercurrent exchange system
A system in which two fluids (liquids or gases) flow adjacent to one another, but in opposite directions, while heat energy or materials move passively from one to the other, from higher to lower temperature or from higher to lower concentration. An example is the countercurrent heat-exchange system located in the legs of many gulls and waterfowl
countercurrent heat exchange
In birds, the direct exchange of heat between warm outgoing blood (such as that travelling to the feet in arteries) to adjacent returning cold blood (such as that returning from the feet in veins) that helps retain heat within the body.
A type of coloration in which an organism is darker on top than below; countershading provides camouflage by reducing the contrast between the top and shadowed underside of an organism so that it appears less three-dimensional.
Interaction in which two birds sing back and forth to each other, alternating their songs.
Together with pratincoles, form the family Glareolidae (17 species). These slender, plover-like ground nesters live in open areas in tropical parts of the Old World, and sometimes cool their eggs or young by partially burying them in sand or by bringing water to the nest in their breast feathers.
courting nests
Nests, usually unlined, produced by male Winter Wrens and some male Marsh Wrens to attract females. A male may build a number of courting nests on his territory. In Marsh Wrens, females may add a lining and lay eggs in the courting nest of their chosen mate.
courtship displays
Displays performed for the opposite sex to acquire a mate of the same species, maintain a pair bond, and/or stimulate and synchronize breeding behavior.
courtship feeding
A behavior in which one member of a pair (usually the male) presents his potential partner with food during courtship or before mating. The food item is sometimes termed a “nuptial gift” and this behavior is an important part of pair bonding in some bird species.
The smaller feathers that partly overlie the flight feathers of the wing and tail at their bases, like evenly spaced shingles on a roof. For more information, see specific types.
Six species of dark, slender, medium-sized birds that forage on seeds and insects; they are members of the New World family Icteridae (blackbirds and New World orioles). All species except the Bay-winged Cowbird are obligate brood parasites
Members of a distinctive family (Cracticidae, 10 species) of songbirds endemic to Australasia. They have stout, straight beaks; loud, melodic calls; and a generalist, crow-like diet of small vertebrates, eggs, insects, and fruits. Cracticids include butcherbirds, currawongs, and the Australasian Magpie.
Toward the head. In practice, may be used interchangeably with anterior (but see separate entry).
cranial kinesis
The ability of the bird’s upper jaw (upper beak) to move upward at the same time that the lower jaw (lower beak) is depressed, an action permitted by the highly flexible craniofacial hinge.
cranial nerves
Twelve sets of paired nerves, each with a specific function, serving the head, neck, and thorax region. Most exit from the medulla oblongata of the brain.
cranial vena cavae
Two large veins (right and left) that gather deoxygenated blood from large veins coming from the wings, head, and neck and deliver it to the right atrium of the heart.
craniofacial hinge
Flexible joint where the upper beak connects to the rest of the skull. Also called the nasal-frontal hinge.
The part of the skull enclosing the brain; the braincase.
An assemblage of the still-dependent young of two or more (usually many) breeding females, attended by one or more adults. Bird species whose young form creches include some pelicans, flamingos, geese, penguins, parrots, jays, and terns; creches of King Penguins and Emperor Penguins may contain several thousand young. Creche formation allows some birds (notably penguins) to conserve energy. It also may decrease predation on the young, and may free parents to spend time foraging in distant areas, thereby allowing them to bring more food back for the young and to prepare themselves for the next breeding.
Active at dawn and dusk.
Tuft of feathers on the peak of the head that either stick up or can be raised.
cricoid cartilages
Two major cartilages that make up the sides and floor of the larynx. Also called the laryngeal cartilages.
A dilation of the lower esophagus that stores food; it is found in many birds that eat dry seeds or fruit containing seeds.
crop milk
Milk-like substance produced by pigeons and doves; it is composed of fluid-filled cells sloughed from the lining of the crop and is regurgitated to feed to nestlings. Crop milk is high in lipids and vitamins A and B, and has a greater protein and fat content than human or cow milk. Also called pigeon’s milk.
In avian conservation, the practice of using surrogate parents (sometimes of a different species) to raise the young of the species being conserved.
cross-fostering experiments
Studies in which young of one species are placed with host “parents” of another species, who then rear their “adopted” young.
The top of the head.
crown group
A special type of monophyletic group (i.e., clade) that includes all organisms descended from the last common ancestor of all living members of a larger clade.
See Conservation Reserve Program.
The lower leg; in birds, supported by the tibiotarsus bone.
Patterning or coloration that allows a bird to remain concealed by blending in to its environment.
General term for birds in the large family Cuculidae. The nearly 50 species of Old World cuckoos are in subfamily Cuculinae, and are all obligate brood parasites. The familiar two-note chime of the cuckoo-clock mimics the song of one of these species—the Common Cuckoo of Eurasia. In the New World, cuckoos are in two subfamilies, Coccyzinae, whose members, all nonparasitic, are termed “New World Cuckoos” and include the Yellow-billed Cuckoo, Black-billed Cuckoo, and Mangrove Cuckoo of North America; and Neomorphinae (the neomorphine cuckoos), whose members are all Neotropical and include three species that are obligate brood parasites.
A crow-sized, arboreal bird with a stout, broad bill; it is the only member of its family (Leptosomatidae) and is found only in Madagascar.
A diverse group of arboreal songbirds, many of which have a shrike-like bill and are slender and (in some cases) barred like cuckoos; however, they are related to neither group. Together with minivets and trillers, they make up the family Campephagidae, found in the warmer parts of the Old World.
cup nest
A nest in the shape of a cup, usually constructed of mud, or small twigs and dried grass, with a depression in the center to hold the eggs; built by the majority of bird species.
Adapted for running.
cursorial theory (of the origin of avian flight)
A theory suggesting that the ancestors of birds evolved to fly by first running along the ground, and then by jumping and leaping, which was augmented by the evolution of wings and feathers, which eventually led to full flight. First proposed by Samuel Williston in 1879. Also called the “Ground-Up Theory.”
The contents of a cell outside the nucleus but within the cell membrane.


A foraging technique in which a bird moves the beak rapidly on the surface of shallow water to pick up small aquatic animals and plant materials; it is used by “dabbling ducks” and a few other species.
dabbling ducks
Ducks (such as Mallards, teal, wigeon, and pintails) that feed by dabbling on the water’s surface, in contrast to diving ducks, which dive under water to search for plant material or aquatic organisms. Dabbling ducks also may tip “bottom up” and reach down under the water to obtain submerged food. Also called puddle ducks.
dark meat
See red fibers.
Darwin’s Finches
See Galapagos Finches.
dawn chorus
The great amount of bird song heard around dawn. At this time the largest number of bird species are singing, and they sing more frequently, and often more energetically and with more variety than at other times of the day. Why birds sing most at dawn, and in different ways than they sing during the day, is not known.
dawn song
Bird song usually given only during the early morning hours; it differs from a species’ normal daytime song.
A stable, persistent, toxic organic compound (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) formed in the body by the metabolism of the organic pesticide DDT (see separate entry). DDE accumulates in fatty tissues and is excreted very slowly, and when concentrations become high it can cause death or other toxic effects such as reproductive failure resulting from eggshell thinning (due to the disruption of calcium metabolism). Thin eggshells severely decreased reproductive success in North American raptors in the 1950s and 1960s, causing populations of most raptor species to plummet.
An organic pesticide (dichloro-diphenyl-trichloroethane) used commonly in the United States from the mid-1940s to the early 1970s to control Mexican boll weevils, gypsy moths, mosquitoes, and other insect pests. DDT is highly persistent in the environment and is taken in by organisms and converted to DDE, a toxic compound that accumulates in fatty tissues and is excreted very slowly. DDT was banned in the United States in 1972, but is still used in other countries, including Argentina, Belize, Ecuador, Guyana, Peru, and Mexico. See DDE for more information.
death rate
The number of individuals in a species or population that die in a given period of time; also called the mortality rate.
Describing trees or shrubs that lose their leaves during part of the year. They generally drop their leaves during the drier season (which in the temperate zone is winter) to conserve moisture, which might otherwise be lost through evaporation.
Level in a food chain or web that consists of organisms (such as earthworms, fungi, and bacteria) that eat dead organisms and waste products, breaking them down into basic nutrients (such as oxygen, nitrogen, and phosphorus) and returning them to the soil for plants to use.
Curved downward. Used to describe beaks such as that of the White Ibis.
deferent ducts
Highly convoluted tubes (also called vasa deferentia) that carry sperm from the testes to (in birds) the cloaca; in birds, the lower portion of each is enlarged to form a temporary storage receptacle for sperm.
definitive plumages
Any of the plumages of a fully mature bird; they may change seasonally, but do not change from year to year as the bird ages. Some species, such as gulls, large raptors, and pelagic seabirds, take several years to reach their definitive plumage.
deflective coloration
Conspicuous markings found on otherwise cryptically colored organisms; on birds, these markings show only in flight or when they are flashed. They may have antipredator or social functions.
delayed plumage maturation
A situation found in some species, in which one sex remains in subadult plumage longer than the other sex.
The statistical study of the size and composition of populations, either at one time or as these factors change over space and time.
Rootlet-like extensions from the cell body of a neuron; they usually receive nerve impulses across a synapse from other nerve cells, but may transmit them as well.
The upwardly projecting knob or peg of the axis (second cervical vertebra); it fits into a hole in the ventral surface of the atlas (first cervical vertebra).
See population density.
density compensation
An extreme form of density inflation in which the total number of individual birds in a setting with low species diversity (such as an island) equals the number of individuals in an analogous setting with high bird species diversity (such as a nearby continent); for this to occur, one or more species must be extremely abundant in the low diversity habitat.
density inflation
A pattern in which a species increases in abundance in locations where competitor species are not found.
density-dependent diversification
The somewhat controversial concept of a feedback loop in which speciation within a group slows down over time as the number of species increases, since the presence of existing species blocks opportunities for new species to arise.
density-dependent factor
A factor that regulates population density in such a way that the magnitude of its effect is determined by the population density. For example, population density is decreased by disease, which has a greater and greater effect as population density increases and thus rates of disease transmission go up. Other density-dependent factors include predation, parasite levels, and competition for resources.
density-independent factor
A factor that affects population density in such a way that the magnitude of its effect does not depend on the population density. Examples include severe weather, natural disasters, and the failure of food supplies.
In birds, the bone forming each side of the lower beak.
deoxygenated blood
Blood whose red blood cells carry very little oxygen. Also called oxygen-poor blood, it is found in all veins except the pulmonary vein.
depolarizing material
A substance that can be used to take a polarized light beam and vibrate it in all directions, creating waves of all orientations, to form an unpolarized beam.
dermal papilla
The portion of a feather papilla formed from dermal tissue; blood vessels extend from the dermal papilla into the developing feather, providing nourishment. This core of tissue remains in a feather follicle throughout a bird’s life, ready to aid each round of feather development after a feather is lost.
The inner layer of the skin; it lies just beneath the epidermis and contains blood vessels, muscles, and nerves.
descending aorta
The continuation of the aorta (after it exits the heart and curves) down through the body and toward the tail.
determinate layers
Bird species that will not lay additional (replacement) eggs if one or more are removed from a clutch during laying. Most determinate layers will lay a new clutch if the entire clutch is destroyed, however. For comparison, see indeterminate layers.
In biology, describes events whose occurrence and/or outcomes are inevitable, based on a certain set of starting conditions. For comparison, see stochastic.
A geographic cluster of similar vocalizations (bird song, human speech, or the sounds of other animals) that is a consequence of those vocalizations being learned. Dialects may exist over very small or very large areas, depending on the details of dispersal and learning.
Describes a condition in which the skull has two openings on each side in the temporal region, posterior to the eye socket. Diapsid skulls have this arrangement, and diapsid reptiles are those reptiles that have diapsid skulls—including thecodonts and their descendants (including birds and crocodiles), snakes, and lizards. Turtles are the only living group of reptiles that are not diapsids.
differential exploitation
A situation in which direct interactions among two or more individuals that seek the same resources are avoided because the individuals use the resources in slightly different ways or use slightly different resources. Differential exploitation may result from the evolution of either morphological or behavioral differences among individuals.
dilution effect
The idea that ecological communities with higher species diversity are more resistant to the spread of pathogens, since there are usually fewer individuals of each particular host species in a high diversity community.
dilution effect hypothesis
The idea that colonial breeding may benefit individuals because their large numbers can overwhelm the consumption capacity of local predators.
dinosaur theory (of bird evolution)
The theory that birds evolved from theropods such as Compsognathus approximately 150 million years ago (proposed by Thomas Huxley in 1868) or from Dromaeosaurs such as Deinonychus approximately 110 to 120 million years ago (proposed by John Ostrom in 1973).
diopter adjustment ring
The ring on binoculars, usually on one of the eyepieces but sometimes on the hinge post, that allows the eyepieces to be focused independently to make up for the differences in visual acuity between an individual’s two eyes.
dip angle
The angle at which the magnetic field lines around the earth contact the earth. The dip angle is 0 degrees at the magnetic equator, and approaches 90 degrees near the magnetic poles.
direct benefits hypothesis
One possible explanation for why females of some species choose the males with the most elaborate ornaments (such as ornate plumage) to copulate with. The explanation applies primarily to females choosing nonpaternal sexual partners, either (1) males for extrapair copulations, or (2) mates in species in which males do not provide parental care or other resources for their offspring (such as territories with food or nesting sites). The direct benefits hypothesis suggests that females may choose the most ornamented males because they are least likely to infect the females with mites, a disease, or some other affliction. Thus the female gains reproductive advantages because her health is not diminished by her mate choice.
direct fitness
An individual’s direct fitness is the portion of its genes that is transferred into the next generation (and beyond) through the production of its own offspring. This contrasts with indirect fitness, which is the portion transferred as a result of an individual’s blood relatives producing offspring.
disjunct range
A range of a species or population that is not continuous, but rather is divided into geographically separate areas.
The movement of an individual bird from one breeding site to another.
displacement activities
Behaviors or actions that seem irrelevant or inappropriate to the current situation. For example, a Herring Gull may stop to preen in the middle of a territorial conflict. Ethologists hypothesize that displacement activities occur because of conflicting motivations or indecision. Some displacement activities have become exaggerated and ritualized into displays.
disruptive coloration
A type of cryptic coloration with patches, streaks, or other bold patterns of color that break up the shape of the organism, catching the eye and distracting the observer from recognizing the whole organism.
Away from the center of the body (fingers are distal to the elbow) or from the origin of the structure (the tip of a feather is distal to its base—where it is attached).
distraction displays
Displays in which a bird or other animal feigns injury or in some other way creates a highly noticeable fuss or disturbance, in order to shift a potential predator’s attention away from the bird’s nest or young. For example, Killdeer give a broken-wing distraction display by dragging and flapping one wing and uttering distress calls, while slowly fluttering along the ground away from the nest or young.
Active during daylight.
divergent evolution
A type of evolution in which different populations of the same species become increasingly distinct from one another over many generations (due to exposure to different ecological factors), eventually diverging into two or more new species.
diversity gradient
A consistent change in species richness along some environmental gradient.
diving ducks
Ducks (such as Canvasbacks, Redheads, scaup, and goldeneyes) that feed by diving under the water’s surface to obtain aquatic plants or animals, in contrast to dabbling ducks, which remain at the water’s surface.
Occurs when members of a socially mated pair of birds become no longer pair-bonded, often to re-pair with different individuals.
DNA (deoxyribonucleic acid)
The genetic material of all cellular organisms and some viruses, forming the chromosomes of these organisms. A molecule of DNA consists of two long strands of nucleotides, held together by bonds between nitrogenous bases on the two strands, with the whole structure twisted to form a double helix. The sequence of the nucleotides is the “genetic code,” which contains the instructions for making proteins, which in turn determine the characteristics of an organism.
DNA fingerprinting
A technique by which the nucleotide sequences of selected portions of the DNA of individuals of the same species are analyzed and compared to determine how closely related the individuals are likely to be. Used in criminology to determine if the DNA of clues such as hair or semen match the DNA of a suspected criminal; also used in biological research to determine the relatedness among individuals whose family histories are not known—sometimes to avoid the breeding of closely related individuals of endangered species.
DNA sequence
The precise order of the nucleotides in a region of the DNA molecule.
DNA-DNA hybridization
An early technique used to determine the degree of similarity (in nucleotide sequence) between two different samples of DNA. It was used to compare the DNA of two different species, to estimate how closely related they are and to hypothesize their evolutionary relationships. Replaced by genome sequencing.
domed nest
A cup nest with an overhead dome that helps conceal the eggs and nestlings.
dominance hierarchy
A ranking system of social status among members of a group; often established and maintained through displays and various aggressive-submissive behaviors, including, on occasion, physical combat. Many dominance hierarchies are linear—A dominates B, B dominates C, and C dominates D—but other arrangements exist.
Toward the back (the vertebral column) of an organism.
A technique in which biologists remove one or several eggs from the nests of indeterminate layers and rear those eggs in the lab while the birds lay replacement eggs and rear the resulting young in the wild. Used by conservationists to increase the population size in declining species such as California Condors and Peregrine Falcons.
down feathers
Soft, fluffy feathers, typically lacking a rachis. Because the barbules lack hooks, the barbs do not cling together, so they trap more air and thus provide extra insulation. Some adult birds have body downs under their contour feathers, and young birds have natal down before molting into their juvenal plumage.
Driving closely behind another vehicle and being pulled forward by air currents moving back over the top of the first vehicle and swirling down and forward, as well as by air currents swirling up and forward from below the first vehicle. Used by race car drivers or cars following trucks on a highway; the energetic advantages are similar to those experienced by a bird flying close to and straight behind another bird, as in a straight-line formation.
A force on a moving object that opposes the direction of travel of the object through a fluid (such as air). Induced drag comes from accelerating air to produce lift; pressure and friction drag are due to air colliding with and moving past the object.
A group of giant, flightless birds that lived in Australia beginning in the Tertiary period, but that became extinct about 10,000 years ago during the Ice Age. They are thought to be neognathous birds most closely related to the Anseriformes (ducks, geese, and swans).
Nonvocal sounds produced by woodpeckers banging on dead trees or other resonant objects with their beaks; most sound like short, emphatic drum rolls, and are given by both sexes to proclaim territory and attract mates.
Singing performance by two individuals. Avian duets usually are given by a paired male and female who may sing in synchrony, overlap one another, or alternate their songs, depending on the species. Most duetting species are tropical, and include some parrots, woodpeckers, antbirds, flycatchers, shrikes, and wrens.
dump laying
Laying an egg or eggs in the nest of a conspecific (or sometimes, a similar species). Females that dump lay usually also build their own nests and incubate their own eggs. Wood Ducks, Hooded Mergansers, and some other cavity-nesting waterfowl commonly dump lay, sometimes resulting in large numbers of eggs in one nest. Also called dump nesting or egg dumping.
The U-shaped first loop of the small intestine, running from the stomach to the jejunum.
The outermost, toughest, and most fibrous vascularized membrane surrounding the brain and spinal cord. Along with the other meninges, it provides sustenance and waste removal for the cells of the brain and spinal cord, which are not served by the circulatory system.
Driving fine particles through the feathers by rolling the body, fluffing the feathers, and wiping the head and bill in a dusty area or by picking up dust and throwing it over the body, after which dust is shaken or preened out; important for feather maintenance and removal of ectoparasites.
dynamic pressure
The pressure of a flowing fluid, or of movement through a fluid such as air or water. You feel dynamic pressure when wind blows against your face.
dynamic soaring
A type of flight in which birds use the gradient in wind speed that exists over the surface of the ocean to travel for long distances without spending much of their own energy. The bird glides down the gradient at an angle, the turns and rises abruptly into the wind, using its momentum to gain height quickly. It then turns and glides down again, crossing the waves in large zig-zags. Dynamic soaring is used most by albatrosses and other large pelagic birds.


See tympanic membrane.
eastern montane forest
North American coniferous forest ecosystem dominated by spruce and fir trees and found below alpine tundra but on the higher summits, ridges, and slopes of the Appalachians from northern New England south through Georgia and Alabama; it hosts relatively few birds.
eclipse plumage
The set of dull-colored feathers worn briefly after the breeding season by some adult birds, such as ducks. In eclipse plumage, male ducks look like females, which do not change much in appearance. Eclipse plumage is acquired by a complete molt after the breeding season, and is soon replaced through a partial molt that produces the brighter colors of the breeding plumage.
ecological (or environmental) niche modeling
A method for predicting the overall distribution of a species based on the environmental conditions at points where the species is known to occur.
ecological constraints hypotheses
A set of hypotheses that each give an explanation for why certain individual birds might forego their own breeding in a particular breeding season, instead acting as helpers at the nest of other breeding pairs (usually their parents or other close relatives). The ecological constraints hypotheses focus on the possible costs to young birds of dispersing from their natal territory; examples include (1) few vacant territories of good quality may be available, (2) few suitable breeding partners may be available, and (3) the birds may have little chance of reproducing successfully until they gain “parenting” experience.
ecological isolating mechanisms
Structural, physiological, or behavioral adaptations that have evolved in species that have very similar niches, and that allow the species to divide up the resources in various ways (such as using slightly different resources or foraging in different parts of the habitat), and thus to coexist. For example, certain wood-warbler species can coexist in spruce forests of northeastern North America even though they all eat similar small insects, because they forage in different parts of trees.
ecological niche
The set of biotic and abiotic conditions that allow a species to occur; sometimes also used in the sense of the role of an organism within its ecosystem or environment.
ecological release
A situation in which the absence of competitors allows a species to occupy an expanded niche compared to locations in which the competitors are present.
ecological release
An expansion of the niche of certain populations of a species, such that a greater breadth of resources such as habitat and food are used, in areas where interspecific competition is lower, as on islands. (In these situations, the niche is “released.”)
ecological speciation
The evolution of reproductive isolation between populations as a result of divergent natural selection in their different locations or environments.
ecological succession
The process by which one association of plants and animals is replaced by another, then that one is replaced by another, and so on until the climax community for that area is reached. The types of communities, and the order in which each succeeds the previous one, is fairly predictable for a given habitat and geographical region. Primary succession begins on a substrate, such as rock, sand, or lava, that has never before supported a community. The process by which a lake gradually fills in to form a bog community is also considered a form of primary succession. Secondary succession begins with bare soil or an existing community.
The study of the relationships between organisms and their environment.
ecoregional planning
Careful, biologically driven planning at the level of ecoregions, with the ultimate goal of preserving the species and ecosystem processes (for example, fire and pollination) that occur within each ecoregion. Ecoregional planning involves geographically delineating ecoregions; cataloging the ecosystems within each ecoregion; identifying and mapping the most important species, communities, and habitats; determining potential threats to species and sites; and then prioritizing species, communities, and the key habitats as to which require the most urgent conservation action. Ecoregional planners then set conservation goals, outline plans, and monitor the results.
Regions of the world that are biologically distinct in terms of climate conditions, topography, soil types, and plant communities; also called physiographic regions.
Both the living and nonliving components of a particular area (including the physical surroundings), as well as the ecological processes that bind them all together (such as decomposition, soil erosion, and water and nutrient cycling). Ecosystems may be small (for example, the ecosystem of a rotting log) or large (a deciduous forest ecosystem).
ecosystem management
Understanding and maintaining entire ecosystems (see separate entry), instead of focusing on particular species or habitat types. The holistic management of habitats and landscapes for the simultaneous purposes of biodiversity conservation and human use.
ecosystem services
The collection of beneficial effects of ecosystems that provide substantial and ongoing economic value to human societies.
A zone of transition between two ecosystems, such as the oak savanna ecotone between the eastern deciduous forests and prairie grasslands of North America. Ecotones host a greater number of species than either adjacent ecosystem, because some species in each of the two adjacent ecosystems frequent the transition zone, and because some species prefer the greater variety of resources found at habitat edges and thus live specifically in those areas.
Parasites, such as flies, ticks, fleas, lice, and mites, inhabiting the exterior of a host’s body.
Describes organisms that must rely on sources of heat outside their own bodies to keep warm; also called cold-blooded.
edge effect
The tendency for areas near the edge of a habitat patch to differ from areas near the center in a number of different ways. For example, areas where two habitats meet fairly abruptly (such as a forest/field boundary) often host a higher number of species than do either of the two adjacent habitats. Other ways in which the edge of a habitat patch may differ from the center include changes in microclimate, such as amount of sunlight or humidity; increased habitat disturbances, such as fire and wind damage; and higher numbers of introduced plants or animals (because these species generally invade from the edge). Forest birds breeding near edges may experience higher rates of predation and/or brood parasitism than birds breeding near the center, because predators such as raccoons, squirrels, and Blue Jays and parasites such as Brown-headed Cowbirds are more common near edges.
The ovum; the female reproductive cell sometimes called the egg cell, both before and just after it is fertilized by a sperm cell.
egg tooth
A short, pointed, calcareous structure on the tip of the upper beak (and sometimes the lower beak as well) that develops in bird embryos shortly before hatching; the embryo rubs and pounds the egg tooth against the inner wall of the eggshell to break it open and hatch. The egg tooth sloughs off or is resorbed by the growing chick within a few days after hatching.
Dissolved inorganic salts that carry either positive or negative charges.
A method of separating large, charged molecules of different lengths or charges (DNA fragments or proteins treated to carry a charge) by their rate of movement through a thin slab of gel in an electric field.
A family (Aepyornithidae) of huge, flightless ratites that lived on the island of Madagascar beginning 10 to 20 million years ago, but were exterminated about 2,000 years ago by human activity. The tallest stood about 10 feet (3 m) and weighed about half a ton.
elevational zonation
A pattern in which related bird species replace one other at different bands of elevation along a mountain slope.
elliptical wings
Short, broad wings having a low aspect ratio; they allow great maneuverability, but do not promote efficient or rapid flight. Elliptical wings are common in birds that live in forests, woodlands, or shrubby areas, such as crows, grouse, quail, and most songbirds.
emarginate tail
Tail shape in which the rectrices become slightly longer from the inside out. Also called notched.
A developing young animal that is still inside its egg or mother; in some animals, especially mammals, refers only to the earlier developmental stages.
The movement of individuals out of a population.
A large, flightless ratite of Australia; it is the only member of its family (Dromiceidae) and is the second largest living bird, next to the Ostrich.
enantiornithine birds
See opposite birds.
Endangered Species Act (ESA)
Federal law passed in the United States in 1973 that commits the government to take action to prevent the extinction of native species and to protect their habitat. It also establishes a procedure to develop a list of threatened and endangered species, identify their critical habitat, and develop and carry out Recovery Plans.
Found only in a particular region; describes a species or other taxonomic group. For example, kiwis are endemic to New Zealand.
endocrine glands
Structures that secrete hormones directly into the blood; the hormones are carried to other parts of the body, where they stimulate or regulate the activities of other glands or organs. The major endocrine glands of birds are the pituitary, thyroids, parathyroids, ultimobranchials, adrenals, gonads, pancreas, pineal, thymus, and cloacal bursa.
endocrine system
Organ system that acts with the nervous system to initiate, coordinate, and regulate body functions, including reproduction and development. It consists of the endocrine glands and their secretions, called hormones.
The fluid that fills the structures forming the membranous inner labyrinth of the inner ear
Having the ability to generate one’s own body heat through metabolic processes; also called warm-blooded. Only birds and mammals are endothermic.
endothermic homeotherm
An organism able to use metabolic heat production to keep its internal body temperature within a restricted temperature range despite conditions in the surrounding environment.
The bone (part of the hyoid apparatus) that supports the tongue; also called the tongue bone.
The surroundings of an organism, including physical features, chemical and energetic factors, and other living organisms.
environmental stochasticity
The tendency of nearly all environments to experience many random (or at least unpredictable) events, such as natural disasters or severe weather. If a population is small and “unlucky” enough to be struck by one of these events, it may be entirely wiped out.
A protein that catalyzes (assists) a biochemical reaction without being consumed in the reaction.
Patches of colored feathers on a bird’s shoulders, such as the red feathers on a Red-winged Blackbird.
See extrapair copulations.
epidermal collar
Structure formed during feather development as a feather papilla elongates
The outer layer of the skin; it protects the inner layer—the dermis—and does not contain blood vessels. In birds, it gives rise to the feathers and to the horny sheath covering the bill, legs, and feet, including the claws.
Plants that grow on other plants but, in contrast to parasitic plants, use their roots for attachment rather than to obtain nutrients from their support plant; examples include many bromeliads (“air plants”) and orchids, and a few cacti.
A disease that spreads quickly among crowded animals, such as viral enteritis (duck plague), which may kill hundreds or thousands of waterfowl.
A unit of geological time. Successive epochs make up a period.
equilibrium theory
Theory proposing that the number of species on an island at any one time represents a balance between the number of new species colonizing the island (immigration) and the number of species becoming extinct. A consequence of this relationship is that smaller islands tend to have fewer species because (1) they tend to have lower immigration rates (they are less likely to be “discovered” by colonists), (2) they have higher extinction rates (the populations are smaller and thus more likely to go extinct due to stochastic factors, and the fewer resources and reduced habitat diversity are more likely to lead to competitive exclusion), and (3) they typically contain a lower diversity of habitat types than larger islands, thereby providing niches for fewer kinds of species. This theory applies only in situations in which speciation is not a major source of new species, for example over short periods of time, or on islands too small or too far from other islands to permit speciation through geographic isolation.
A unit of geological time. Eras are divided into periods.
See red blood cells.
See Endangered Species Act.
Thin, straight muscular tube carrying food from the pharynx to the stomach. It is lined with mucus-secreting glands that moisten food to ease its passage, but contains no digestive glands.
The study of animal behavior, primarily from a proximate approach, by comparing similar behaviors in related species to understand how certain behaviors evolved, and by investigating releasers and instinctive behaviors and the underlying physiological processes. Some of the earliest animal behaviorists (primarily Europeans) were concerned mainly with ethology, and the two pioneers of the field, Konrad Lorenz and Niko Tinbergen, laid the groundwork for animal behavior studies carried out today.
eustachian tube
See auditory tube.
The changes in a lake, estuary, or slow-moving stream when it receives excess plant nutrients (especially nitrates and phosphates) through the erosion of soil, run-off from adjacent land, and discharges from industries and sewage treatment plants. Some sources are natural, but many are caused by humans, in which case the changes are sometimes termed cultural eutrophication. One important change is the dramatic increase in the growth of plants and (especially) algae, which can choke out native plants. Then, as the masses of plants and algae die and sink to the bottom they decompose, depleting the water of oxygen, and thus few aquatic organisms can continue to survive. In addition, some algae may produce toxins, dyes, or odors that decrease water quality.
evaporative cooling
The loss of heat via the energy required to evaporate water.
eventual variety
Pattern of singing in which a bird repeats one song type many times before switching to a different type, which it then repeats many times, and then switches to another type, and so on.
A change over time. The evolution of living things is the set of cumulative changes in the characteristics of a species or population over successive generations that result from natural selection acting on the genetic variation among individuals.
evolutionarily stable strategy
An equilibrium strategy or behavior that, once established in a population, cannot be displaced by selection for an alternative strategy that gives individuals higher fitness.
In avian biology, bird species that dig their own nest cavities or tunnels, either in sandy soil (such as Belted Kingfishers, Bank Swallows, bee-eaters, mot-mots, and Crab Plovers) or in wood (woodpeckers).
excretory system
Organ system consisting of the kidneys and their ducts, the ureters (in the rheas of South America, but in no other birds, a urinary bladder is also present); it removes toxic nitrogenous wastes from the blood by producing, storing, and excreting urine. In birds, the urine is composed of uric acid. Also called the urinary system; it is often considered together with the reproductive system as the urogenital system.
exit pupil
The space through which the light beam exiting the eyepiece of binoculars or a telescope passes; this is, in effect, the hole through which the observer looks. The diameter of the exit pupil is calculated by dividing the size of the objective lens by the magnification; the larger the exit pupil, the brighter the image.
exploded lek
A loose aggregation of males that engage in competitive mating displays towards visiting females; exploded leks are more spread out in space than are classical leks.
exploitation competition
Competition among species that results from indirect interactions, usually when both species require the same set of limited resources such as food or nest sites.
A muscle that pulls one bone away from another bone.
external ear
The portion of the ear containing the external ear canal and the eardrum (tympanic membrane.
external ear canal
The channel through which sounds enter the ear; it leads from outside the body to the eardrum.
extinction events
Large-scale extinctions of many species; they have occurred periodically throughout Earth’s history.
extinction vortex
A negative feedback loop affecting small populations, in which genetic and demographic factors compound one another and worsen with every successive generation.
extra-embryonic membranes
Membranes that protect and nourish the growing embryo, but do not become part of the adult body. The four main ones in the avian egg are the yolk sac, amnion, allantois, and chorion.
extra-pair copulation
Matings between two members of a population who are not pair bonded, usually when each is socially pair-bonded to a different mate.
extra-pair fertilization
Fertilization of eggs resulting from matings between two members of a population who are not pair bonded.
extra-pair young
In a bird species with male-female social pair bonds, offspring that are sired by a male that is not the female’s social mate.
eye ring
A circle of distinctively colored feathers or skin surrounding the eye.
The eye; in birds it is flat to tubular, in contrast to the spherical eyeball of mammals. Its outer layer is the sclera, and the entire eyeball sits in the socket and is protected on its exposed side by the eyelids and nictitating membrane.
eyebrow stripe
A distinctively colored line running from the upper beak toward the back of the head, located ventral to the boundary of the forehead and crown; also called the superciliary line.
A distinctively colored line that passes through the eye.


facial disc
Flat, relatively round, forward-facing part of the head of owls; it probably funnels sounds into the bird’s ear openings. Also spelled facial disk.
facial nerve
The seventh cranial nerve; it carries motor signals to muscles that protrude the tongue, lower the lower beak, constrict the neck, and (in birds) tense the columella ear bone. It also may carry some taste sensory input from the tongue.
facultative migration
A pattern in which the number, distance and timing of migrating individuals in a population changes from year to year in response to varying environmental conditions.
facultative partial migration
See partial migration.
facultative sex-ratio manipulation
The still somewhat mysterious ability of some female birds to change the sex ratio of the eggs that they lay, in response to varying social or environmental conditions.
Two species of arboreal songbirds endemic to the Oriental zoogeographic region; they are named for the brilliant blue and black plumage of the males, and feed primarily on figs and other fruits. Together with leafbirds, they make up the family Irenidae.
An Australasian family (Meluridae, 26 species) of cooperatively breeding, wrenlike birds with long, cocked tails.
false sunbirds
Members (with asities) of the family Philepittidae, endemic to Madagascar. These two species of suboscine passerines feed on insects and nectar.
Level of classification of organisms above “genus” and below “order”; similar genera are placed in the same family. The scientific names of bird families end in “idae” (for example, Corvidae).
See fixed action pattern.
Connective tissue binding together hundreds or thousands of muscle fibers to form a skeletal muscle; it may be in the form of bandlike tendons or broad, shiny sheets called aponeuroses.
fat bodies
In birds, yellowish fat deposits laid down just under the skin, usually in individuals storing fat in preparation for migration; the most conspicuous fat bodies lie over the abdomen and in the depression formed anterior to the breast muscles where the clavicles fuse to form the wishbone, and are visible in a hand-held bird with the feathers parted. Researchers may use the degree of fat accumulation in a migrant as an indication of its energetic condition and potential to continue migration.
fault bars
Transverse regions on a mature feather with slightly different coloring that correspond to each day of feather growth.
feather comb
See pectinate claw.
feather follicle
A small, epidermis-lined pit in the skin of a bird from which a feather grows and to which it remains attached.
feather papillae
Small bumps covering the surface of the skin during embryonic development. They consist of a core of dermis and a covering of epidermis, and each will eventually form an embryonic feather. Also simply called papillae.
feather sheath
A thin, cylindrical tube of keratin surrounding and protecting a developing feather. It eventually breaks open to let the mature feather unfurl.
feather tracts
Areas of a bird’s skin where feathers are attached; also called pterylae.
fecal sac
A tough, flexible bag enclosing the feces of most passerine nestlings; it allows the parents to remove and dispose of the feces more easily—parents sometimes grab the fecal sacs as they emerge from a nestling’s cloaca. Many parents carry the sacs some distance from the nest and drop them, but others eat them.
See birth rate.
female-defense polygyny
Mating system in which males compete fiercely for control of clusters of nesting females. In some species, such as Montezuma Oropendolas of Central America, a male dominance hierarchy results with the top few males securing most of the matings. Because males do not help to rear the young, this system may evolve when male parental care is less important to the survival of the young than are safe nesting sites or rich food supplies.
Bone that supports the upper hind limb (thigh) of many vertebrates, including birds and humans.
fertility insurance
Mating with multiple males to help ensure that at least one partner is sexually fertile.
The thinner of the two lower hind limb bones in many vertebrates, including humans; in birds, the fibula is reduced and present only as a thin, needlelike bone running two-thirds of the way down the side of the tibiotarsus.
field metabolic rate
The rate of heat production by a bird that is engaging in a natural daily routine of activities; a measure of the total energy required for physiological maintenance, movement, digestion, heating and cooling, etc.
field of view
The width of the area visible (usually at 1,000 yards from the observer) through binoculars or a telescope. If the field of view is labeled in degrees, multiply degrees by 52.5 to get the width in feet. 2. The view attained by a particular species, due to the placement of its eyes; also called an organism’s visual field.
Hairlike but relatively stiff feathers having a rachis but few or no barbs (any barbs are present only at the tip). In the skin next to their follicles they have sensory receptors, which allow them to monitor movement within the feather coat.
An individual’s degree of success at contributing its genes to the next generation; often measured as the number of its offspring that survive to reproduce.
fixed action pattern (FAP)
A behavior that occurs in complete form each time the animal encounters the releasing stimulus—even upon the animal’s first exposure to that stimulus. An FAP may be a simple or complex behavior, or a series of behaviors, but once begun, it is played out to the end regardless of any response that occurs or any intervening stimuli.
flagship species
A charismatic species that is typical of a given habitat and is the target of special public and/or conservation attention.
See oral flanges.
The side of a bird, dorsal and caudal to the leg.
Term commonly used to describe the time at which nestlings that are reared in the nest leave the nest, even though their flight abilities may not yet be well developed. But, the term is sometimes used to describe the time at which a young bird has finished acquiring its first complete set of flight feathers—generally the time at which it is capable of flight. The term is used less often in precocial species that leave the nest shortly after hatching, but sometimes it refers to the time at which they begin to fly. “Fledging” may also be used to refer to the process of reaching the moment of fledging.
fledging period
The period of time from hatching to the moment of fledging (see separate entry).
A young bird that has recently fledged (see separate entry for fledging).
A muscle that pulls one bone toward another bone.
flight feathers
The remiges of the wings and rectrices of the tail.
flight songs
Songs given by birds during flight; they are particularly common among birds of open areas, such as grasslands and the tundra, where few perches are available. Singing from higher up generally increases the distance over which a song can be heard. Nevertheless, some species that do have ample perches, such as the forest-dwelling Ovenbird, also give flight songs; these usually begin with a jumble of notes that appear to draw attention to the singer, and then proceed with the bird’s normal song. Their function is unknown.
Animals, generally males, that do not hold territories or form pair bonds, but cruise around areas containing territorial individuals, waiting for a chance to take over a territory or sneak a copulation with a paired bird.
A family (Dicaeidae, 43 species) of small, busy, noisy songbirds, primarily of the Oriental zoogeographic region, that forage high in trees on berries, nectar, and insects.
flush-pursuit foraging
A foraging strategy in which birds first scare prey (usually insects) out of hiding and then chase and capture them.
General routes used by large numbers of migrants of many species.
A small, epidermis-lined pit in the skin of a bird, from which a feather emerges and to which it is attached.
food chain
The sequence in which organisms in an ecosystem feed upon other organisms. Most food chains consist of producers, consumers, and decomposers. A food chain is a fairly linear and simplistic model of feeding relationships in communities, which are more realistically represented as food webs because of the numerous interconnections among species and levels.
food web
See food chain.
In birds, refers to the portion of the leg distal to the tibiotarsus bone, and has two sections. The upper section is supported by the tarsometatarsus bone, which does not touch the ground when the bird walks; the lower section consists of the phalanges of the toes, upon which the bird walks.
foraging guild
A group of species in a community that use similar foraging behaviors and/or which feed on a generally similar type of food resource.
foramen triosseum
An opening at a bird’s shoulder joint, formed by the junction of the scapula, coracoid, and clavicle bones. This hole acts as part of a pulley system that allows the force of the supracoracoideus muscle to be redirected
The anterior portion of the brain; it consists of the two cerebral hemispheres with the olfactory lobes at their anterior ends.
The front of the head, from the crown to the base of the bill.
forked tail
Tail shape in which the rectrices become abruptly longer from the inside out.
An ordered arrangement of a group of birds in flight, such as V-shaped flocks of geese or single lines of Brown Pelicans and cormorants.
founder event
The loss of genetic variation that occurs when a new population is established by a very small number of individuals immigrating from a much larger source population.
Also known as pitch, the frequency is the rate at which a sound causes the air through which it is moving to compress and thin (one compressing and thinning is called one cycle), and is measured in cycles per second, or Hertz (Hz). The more cycles per second, the higher the frequency, and the higher the pitch.
fright molt
See shock molt.
A family (Podargidae, 14 species) of nocturnal forest birds of the Oriental and Australasian regions that resemble their smaller nightjar relatives in both cryptic appearance and behavior.
frontal plane
A plane, usually horizontal, through an organism, dividing the body into dorsal and ventral portions.
Feeding mainly or exclusively on fruits.
functional response
A change in the amount of a certain type of prey taken by a predator, as a result of a change in the population density of that prey. For example, as insect populations in an area increase during spring, American Kestrels may begin to eat more insects and fewer voles.
fundamental niche
The full range of environmental conditions and resources a bird could possibly occupy and use without regard to other limiting biological factors such as the presence of competitors, pathogens, or predators.
V-shaped bone of the pectoral girdle of birds, formed by the fusion of the right and left clavicles with a small interclavicle bone; also called the wishbone.
Describes an egg that is slightly longer than subelliptical; also called biconical or long subelliptical.


Galapagos Finches
A group of 15 finch species in the family Emberizidae living in and near the Galapagos Islands. They are a classic example of adaptive radiation, as they have a wide array of beak sizes and shapes and are all thought to have evolved from a common ancestor. Also called Darwin’s Finches.
gall bladder
Small organ for storing bile; it is located under the liver and is not present in all birds.
gallinaceous birds
Grouse, quails, turkeys, pheasants, and all other birds in the order Galliformes; includes domestic chickens.
game theory
Mathematical cost/benefit models of alternative behavioral strategies involving conflict versus cooperation.
The ova (egg cells) and sperm cells.
gamma diversity
The overall species diversity across all the sites and habitats within a broader region.
ganglia (singular, ganglion)
Aggregations of nerve cell bodies; they form nerve centers outside the central nervous system (in the peripheral nervous system).
gap analysis
In conservation, the comparison of maps of species distributions with those of protected areas to identify critical but still unprotected areas that merit additional conservation action.
gape flanges
Brightly colored enlargements around the base of the bill in nestlings of species in which parents feed the chicks.
gape size
Size of the mouth opening through which food items must pass.
In avian biology, begging behavior of young birds that begins shortly after hatching in which they open the mouth widely; may be accompanied by a begging call. Given by altricial young and those precocial young whose parents feed them.
gas exchange
The movement of gases between an organism and the environment; for example, in the lungs of many organisms including birds, the blood takes up oxygen from the air and discharges carbon dioxide and water.
gastric cuticle
Leathery or sandpaper-like material that forms the lining of the gizzard; it is a combination of carbohydrate and protein secreted by glands in the wall of the gizzard. Also called koilin.
gastrointestinal tract
See alimentary canal.
The sequence of base pairs within a molecule of DNA that codes for one specific protein.
gene flow
The movement of genetic material between populations. In mobile animals, gene flow generally occurs as individuals emigrate, immigrate, or breed with individuals from other populations. In organisms such as plants and fungi, gene flow occurs as spores, pollen, or seeds are carried by water, wind, or animals.
gene pool
All the genes existing in a population at a given time.
In biology, an organism that is able to use a wide range of some type of resource; for example, animals with generalist diets eat many different types of foods.
genetic benefits
Benefits of mate choice that stem from the high-quality genetic variants inherited by offspring of partners that choose genetically high-quality mates.
genetic bottleneck
The loss of genetic diversity experienced by most populations as they become very small. Such loss occurs for three main reasons
genetic drift
Random changes in genetic diversity in a population over time, often leading to the loss of genetic diversity when populations are small.
genetic monogamy
A mating system in which a female mates with only one male.
genetic rescue
Introduction of individuals bearing novel genotypes from elsewhere into a population suffering from low genetic diversity, thereby increasing that diversity.
genetic structure
The relative proportions of individuals with different genetic types—usually noted for a given population.
genital system
The reproductive system.
The complete set of genetic material—DNA—within an individual bird.
genus (plural, genera)
Level of classification of organisms above “species” and below “order.” Genus is always capitalized, and is underlined or printed in italics.
geographic isolation
A barrier to interbreeding between two populations that arises from them being separated in space, often with some kind of geographic barrier to movement between them.
geographic range
The geographic area within which a species or population generally remains at a particular time of year; a species may have different breeding and nonbreeding ranges. Also called the range.
Consumption of soil, clay, or sand.
germinal spot
In avian biology, the light-colored site on the egg yolk where the embryo will eventually develop. The germinal spot sits atop a cylinder of light-colored yolk that stretches from the yolk’s core to its surface.
ghost lineages
An evolutionary lineage that is inferred from a phylogeny, but for which no fossil evidence has been found.
The lower part of the bird’s two-part stomach; it is rounded and has a tough lining and thick, muscular walls, often with internal ridges. It grinds and softens foods, and in birds that eat seeds, the gizzard has more muscular walls than in birds that eat meat. Seed-eating birds may eat grit or small stones, which reside in the gizzard to aid in grinding. In birds that eat fish or other meat, the gizzard molds indigestible material, such as bones and feathers, into compact balls (pellets) that are then ejected through the mouth.
A foraging technique in which a bird takes insects and other small invertebrates from the surface of vegetation or other substrates. In perch gleaning, practiced by many wood-warblers and other species, a bird grabs prey without flying from its perch. In sally gleaning, practiced by birds such as Red-eyed Vireos, chickadees, titmice, and some small flycatchers, the bird sits still and watches the surrounding vegetation until it sees an insect move, then flies out and grabs it from the surface. In hover gleaning, practiced by kinglets, phoebes, and Great Crested Flycatchers, among others, the bird hovers while taking food from the surface of vegetation.
glenoid fossa
In birds, a cup-shaped depression formed where the coracoid and scapula meet; it receives the rounded end of the humerus, forming a ball-and-socket joint that enables the humerus to rotate freely around the shoulder joint.
glial cell
See neuroglia.
glide angle
The angle at which a gliding bird descends through the air.
Unpowered flight (no thrust is provided) in which the flying object loses altitude. In a bird or other animal, flying without flapping the wings or limbs, while losing altitude (as compared to soaring, in which the animal rises).
gliding flight
Unpowered flight, as in birds that are flying without flapping their wings.
globular nest
A spherical dome nest with a top that completely encloses the nest; usually entered through a hole on the side. Examples include the nests of Cactus Wrens, Black-billed Magpies, and Southern Penduline-Tits.
glossopharyngeal nerve
The ninth cranial nerve; it carries sensory and motor information between the brain and the tongue, pharynx, esophagus, and throat. It also carries motor output to the salivary glands.
Small, slit-like opening to the larynx; it is surrounded by fleshy folds whose muscles regulate the passage of air into the respiratory system.
glycogen body
A gelatinous mass of neuroglial cells rich in the nutritive sugar glycogen; it is located in the rhomboid sinus, and its function is unknown. The rhomboid sinus and glycogen body are unique to birds.
The primary sexual organs, the testes and ovary, which produce, respectively, sperm and eggs. They also are endocrine glands, secreting the sex hormones testosterone, estrogen, and, from the ovary only, progesterone.
The name of the ancient southern supercontinent of 600-180 million years ago that included most of present-day Africa, Antarctica, Australia, India, Madagascar, New Zealand, and South America.
The southern land mass formed 200 million years ago when Pangea split into two large land masses. It consists of present-day South America, Africa, Madagascar, India, Australia, New Zealand, and Antarctica.
good genes benefits
The fitness advantage that individuals (usually females) gain by selecting mates on the basis of the genetic qualities that they will pass on to their mutual offspring.
good genes hypothesis
One possible explanation for why females of some species choose the males with the most elaborate ornaments (such as ornate plumage) to copulate with. The explanation applies primarily to females choosing nonpaternal sexual partners, either (1) males for extrapair copulations, or (2) mates in species in which males do not provide parental care or other resources for their offspring (such as territories with food or nesting sites). The good genes hypothesis suggests that the most ornamented males have genes that increase their own survival in some way (for example, they may have greater skill at foraging, avoiding predators, or obtaining good territories), and that females choose them because then their own male and female offspring may inherit those traits and have an increased chance of surviving and producing offspring of their own.
graduated tail
Tail shape in which the rectrices become abruptly longer from the outside in.
The attractive force between two masses of matter; this force, for example, tends to draw objects toward the center of the earth.
gray matter
Darker-colored tissue (compared to white matter) that makes up much of the brain and spinal cord. It consists of numerous nuclei, which are collections of nerve cell bodies, and is found at the core of the spinal cord and in the outer areas of the brain.
A foraging technique in which an animal bites off clumps of grass or other vegetation; used by geese, antelope, and others.
Great American Interchange
The exchange of animals that commenced about 3 million years ago when a land bridge first connected the previously isolated continents of North and South America.
greater coverts
The feathers partly overlying each remex on the upper surface of the wing. A greater primary covert overlies each primary feather, and a greater secondary covert overlies each secondary feather.
gross primary productivity
See primary productivity.
ground speed
The speed of a moving object (such as a flying bird) in relation to the ground; it is equal to the bird’s speed with respect to the air (the air speed) plus or minus the wind speed, depending on the direction the bird is flying with respect to the wind.
A family (Brachypteraciidae, 5 species) of solitary, terrestrial insect-eaters with stout bills, short wings, and moderately long legs and tails. They are endemic to Madagascar and are declining in number dramatically.
ground-up theory (of the origin of avian flight)
See cursorial theory.
group selection
The controversial idea that natural selection can sometimes act collectively via benefits realized by entire groups of organisms, as opposed to the narrower fitness benefits realized by selection acting only on individuals.
The accumulation of seabird droppings, particularly at a breeding colony, dried to a hard, crusty state. Guano is often mined and used as fertilizer.
A group of species that use components of their habitat or resources in a similar way; for example the “insectivore guild” or the “hole nesting guild” of a woodland habitat.
Gregarious, chicken-like birds with distinctively spotted and striped plumage; they are often domesticated or found in zoos. The six species are endemic to Africa and form the subfamily Numidinae, of the family Phasianidae.
gular fluttering
Opening the bill wide and vibrating the thin, expansive gular membranes of the throat, in order to dissipate heat. This cooling method is used by pelicans, cormorants, herons, owls, and nighthawks.
gular region
Upper part of the throat, just below the chin.
See alimentary canal.


The physical surroundings in which an organism lives. It consists of physical factors, such as light, temperature, and moisture, as well as living organisms, such as plants and animals. Habitats are often characterized by a dominant plant type or physical feature, such as a grassland habitat or stream habitat.
habitat conservation plan (HCP)
A plan that must be submitted to the U. S. Fish and Wildlife Service by anyone who applies for a permit to destroy endangered species or their habitats (as allowed under a 1982 amendment to the Endangered Species Act). The plan must specify the steps that the applicant will take to minimize the number of individuals killed and to minimize the impact on the species as a whole, and also must explain why other alternatives are not feasible.
habitat corridor
An area of habitat that connects larger patches of that habitat and through which species dependent on that habitat can move.
habitat fragmentation
The process by which a large, continuous habitat is broken into a number of small, isolated patches by activities such as development, logging, or farming.
habitat generalists
Species that can live and breed successfully in a wide range of different habitats. If all else is equal, these species are less likely to go extinct than species that can live and breed only in one or a few specific types of habitats (habitat specialists).
habitat imprinting
The process by which a young animal, especially a bird, learns the characteristics of appropriate habitat by observing its surroundings while it is still living within its parents’ territory. When adult birds eventually settle on their own territories, most choose to live and/or breed in areas similar to those in which they were raised.
habitat island
An isolated patch of a given habitat that is surrounded by other, potentially inhospitable habitats.
habitat isolation
A barrier to interbreeding between two populations that arises from them being found in different habitats, even if otherwise they are in close proximity.
habitat richness theory
The idea that smaller islands generally hold fewer species than larger islands because they tend to have fewer different habitats, and thus species immigrating to smaller islands are less likely to find suitable habitat than those colonizing larger islands, and are more likely to perish (or leave).
habitat specialists
Species that live and breed successfully in only one or a few specific types of habitats. If all else is equal, these species are more likely to go extinct than species that can live and breed successfully in a wide range of habitats (habitat generalists).
habitat specificity
How wide a range of different habitat types in which a species can live and breed successfully.
The permanent loss of a response as a result of repeated stimulation without reward or punishment; learning to ignore unimportant stimuli.
The technique of introducing young, captive-raised birds of prey, especially falcons, to appropriate habitat by releasing them from an enclosure that serves as an artificial nest and in which biologists continue to place food until the bird has learned to hunt on its own.
The first toe, composed of two phalanges. In nearly all birds, it points backward. The hallux is well developed in perching birds and reduced or absent in many running birds.
A stork-like water bird whose shaggy, crested nape and stout, tapering bill make the head appear hammer-shaped. It builds an enormous mound nest. It is endemic to Africa, and is the only member of its family, Scopidae.
Hamilton’s Rule
The formula that describes when an individual should forgo its own reproduction and instead help its relative reproduce. One formulation of this idea suggests helping should be favored when rB > C, where r is the genetic relatedness of the individual and its breeding relative, B is the additional number of offspring this helping behavior will add to the relative’s reproductive success, and C is the cost to the helper in the form of offspring that they can’t produce themselves if they invest instead in helping their relative.
See manus.
handicap principle
The somewhat controversial hypothesis that females of some species have evolved mating preferences for males who display very costly and exaggerated ornaments or behaviors, since these costs ensure that only particularly high quality males can afford these so-called handicaps.
Emerging from the egg. A clutch may hatch synchronously (all at about the same time—see synchronous hatching) or asynchronously (over a period of several days—see asynchronous hatching).
A newly hatched animal.
Hawaiian Honeycreepers
A group of 32 species, many of them extinct, in the subfamily Drepanidinae, within the family Fringillidae. These small, colorful birds have a wide array of beak shapes and are a dramatic example of adaptive radiation, as they all are thought to have evolved from a common ancestor.
A foraging technique in which a bird sits very still on a high or exposed perch, and when it sees an insect, flies out and snatches it in midair, returning to the same or a nearby perch; used by many flycatchers, kingbirds, bee-eaters, waxwings (sometimes), and some woodpeckers, among others.
See habitat conservation plan.
The compass direction in which a bird is pointing its beak and propelling itself through the air. Because of crosswinds, the heading may not be the actual direction that the bird is progressing with respect to the ground. Also applies to other flying animals and aircraft.
heel pad
Calloused enlargement of the upper end of the tarsus (at the heel), found in the nestlings of many cavity nesters, such as woodpeckers and trogons. It is thought to reduce abrasion of the tarsus from the rough lining of the nest cavity.
helpers at the nest
Adult birds that are not currently breeding themselves, but assist other breeding pairs (usually, but not always, their relatives—especially their parents) in rearing their offspring. Some helpers are birds whose own breeding attempts have failed, whereas others are unpaired birds or those without territories; helpers often breed on their own in subsequent years. Bird species in which helpers are common are called cooperative breeders.
hepatic portal system
Pattern of circulation in which blood from the capillary beds of the small intestine is brought by the hepatic portal vein to the capillary beds of the liver, for further processing.
hepatic portal vein
Large vein carrying blood from the upper part of the small intestine, where the blood has absorbed digested nutrients, to the liver, where it undergoes further chemical processing before returning to the heart. “Portal” veins carry blood between two capillary beds—in this case, the capillary beds in the small intestine and the liver—instead of between a capillary bed and the heart.
hepatic veins (left and right)
Two large veins that carry blood from the capillary beds of the liver to the caudal vena cava near its entry into the right atrium of the heart.
hepatoenteric ducts
In birds that lack a gall bladder, ducts that transport bile from the liver to the small intestine.
Organisms that eat primarily plants.
The proportion of the variation in a trait that is due to underlying genetic variation.
Hertz (Hz)
A measure of the frequency of a sound, in cycles (of compression and thinning of the air) per second.
heterocoelous centrum ends
The saddle-shaped, interlocking ends of the centrum (main body) of avian cervical vertebrae. Because the anterior end is concave in a lateral direction and the posterior end is concave in a dorso-ventral direction, articulating vertebrae can rotate freely against one another, allowing the neck to be highly flexible.
heterodactyl foot
Foot arrangement in which the third and fourth toes point forward and the hallux and second toe point backward; found in trogons.
Describes individuals whose two sex chromosomes are different; in birds, the chromosomes are called ZW and heterogametic individuals are females, but in mammals, the chromosomes are called XY and heterogametic individuals are males. Heterogametic individuals are capable of producing two different types of gametes (eggs or sperm)—one with one type of sex chromosome and one with the other. For comparison, see homogametic.
heterospecific attraction
A behavior in which individuals of one bird species use the presence of individuals of another species as an indicator of the quality of a habitat patch in which to forage or breed.
Possessing two alleles that are different for a given gene; for example, having one allele for blue eyes and one allele for brown eyes. (Each allele comes from a different parent.) For comparison, see homozygous.
high-aspect-ratio wings
Wings that are long, narrow, and unslotted; the length derives primarily from the lengthened inner wing (as compared to high-speed wings, in which the length results primarily from the long outer wing). High-aspect-ratio wings are highly efficient at producing lift at relatively high flight speeds, but they are difficult to maneuver, especially during take-offs. They are found in a few seabirds that are highly specialized for dynamic soaring over the ocean, such as albatrosses, shearwaters, petrels, and some gulls.
high-speed wings
Wings that are tapered, pointed, and in many cases sweptback, with unslotted primaries and a high aspect ratio; found in birds such as falcons, swifts, swallows, terns, ducks, and many shorebirds. High-speed wings allow good control and high speeds, but are energetically expensive to use because the bird must flap constantly.
The posterior portion of the brain; it includes the cerebellum and medulla oblongata (brain stem).
hinge post
Central rod of binoculars, around which the two barrels pivot.
An odd-looking, leaf-eating bird that nests in branches overhanging lakes and slow-moving streams in the Neotropics; it is the only member of its order and family (Opisthocomidae). To avoid predators, nestlings may temporarily leave the nest and clamber about in trees, aided by small claws on their wings.
A combination of the Nearctic and Palearctic zoogeographic regions; the Holarctic encompasses the Northern Hemisphere north of the tropics.
The ability to maintain relatively constant physiological conditions within the body.
homeostatic mechanisms
Mechanisms that act in various ways to preserve balance. For example, various physiological factors keep the body temperatures of birds and mammals relatively constant. And, various ecological factors keep many populations at a density that fluctuates around a fairly stable level.
An organism that maintains its internal body temperature at a constant or nearly constant level regardless of environmental conditions.
Describes animals that are able to keep their internal body temperature constant even when the outside temperature varies. In birds and mammals, the hypothalamus monitors body temperature and, if required, triggers responses that warm or cool the individual to bring it back to normal body temperature.
homing ability
The ability to return to a specific place. For example, homing pigeons can return to their lofts when released from great distances away.
Describes individuals whose two sex chromosomes are the same; in birds, the chromosomes are called ZZ and homogametic individuals are males, but in mammals, the chromosomes are called XX and the homogametic individuals are females. Homogametic individuals are capable of producing only one type of gamete (eggs or sperm)—one that contains the same type of sex chromosome as the parent. For comparison, see heterogametic.
Possessing two identical alleles for a given gene; for example, having two alleles for blue eyes. For comparison, see heterozygous.
A huge, diverse Australasian family (Meliphagidae, 181 species) of dull-colored, arboreal songbirds with medium-length, curved bills. Honeyeaters have a distinctive brush-tipped tongue for gathering nectar and are important pollinators, but they also eat insects and fruits. They feed busily, and often congregate at flowering trees.
A nonpasserine family (Indicatoridae, 17 species) of the warmer parts of the Old World, whose members are peculiar in their ability to digest wax, especially beeswax, in addition to their insect prey. Some honeyguides lead humans or other mammals to bee nests, and then eat the wax that is exposed as the mammal breaks open the nest.
Tiny hooks found on each of the barbules that branch from the distal side of each barb of a contour feather; hooklets catch onto the barbules branching from the proximal side of the next barb (toward the feather tip), lightly holding the barbs together to form a smooth, continuous vane.
Chemical substance secreted into the blood and thus carried to other parts of the body, where it may stimulate or regulate the activities of glands or organs. Hormones are the messengers of the endocrine system.
A family (Bucerotidae, 55 species) of large, toucan-like birds of the Oriental and Afrotropical regions; hornbills have long tails and huge, down-curved bills topped by a distinctive casque. They are famous for the female’s nesting behavior—she seals herself inside the nest cavity with the eggs and young.
horns of the hyoid
Part of the hyoid apparatus; the two bones on each side of the hyoid that extend backward (caudally) from the tongue bone, running beneath the skull and then curving upward around the back of the head.
House Finch eye disease
See mycoplasmal conjunctivitis.
hover gleaning
See gleaning.
A type of flight in which an aircraft or flapping animal remains suspended in air in one place. In birds hovering usually is achieved by positioning the body nearly vertically and beating the wings more or less horizontally, producing just enough forward thrust to balance wind speed, and just enough lift to compensate for gravity. Hovering is an energetically expensive undertaking.
humeral patagium
A flap of skin extending from the brachium to the trunk of the bird.
The bone supporting the brachium (upper arm or wing).
Humphrey-Parkes Nomenclature
The system of naming plumages and molts most commonly used by ornithologists today. In this system, the plumage worn for the longest time each year (the nonbreeding plumage), usually produced by a complete molt, is called the bird’s basic plumage, and other plumages are called alternate and supplemental. In addition, molts are named for the type of plumage they produce, not the feathers they shed. In the traditional system, a bird’s breeding plumage is considered to be the main plumage.
hybrid index
A number derived from measuring a series of traits or genes that describes the proportion of an hybrid individual’s ancestors that belonged to each parental species.
hybrid zone
A geographic area where two species (or distinct populations) come into contact and hybridize.
Breeding that occurs between two individuals of different species.
The study of water
hyoid apparatus
A V-shaped unit composed of bones and cartilage, located between the two halves of the lower jaw. The hyoid apparatus consists of the tongue bone (entoglossal) and the horns of the hyoid, which, respectively, support the tongue and the muscles that control tongue movement.
The dramatic increase in the amount of food that birds consume as they prepare to migrate; because these birds eat more than their body requires in the short term, they store body fat to be metabolized for energy during migration.
Condition in which the body temperature rises a few degrees above normal; if the animal cannot bring its temperature down, it soon dies.
hypoglossal nerve
The twelfth cranial nerve; it combines with the vagus (tenth) and glossopharyngeal (eleventh) cranial nerves to form a combined trunk that controls movement of the tongue, larynx, trachea, and syrinx.
The ventral portion of the brain region between the forebrain and midbrain. The hypothalamus plays a major role in hormonal control of body processes by using special neurosecretory neurons to control the pituitary gland, which extends from a stalk below the hypothalamus.
Condition in which the body temperature drops below normal. If the animal cannot bring its temperature up, it soon dies.
A tentative explanation for an observation or phenomenon; hypotheses are usually stated in a way such that they can be tested via the scientific method.
See Hertz.


Important Bird Area. See Important Bird Areas Program.
ideal-free distribution
A theoretical model for how birds might distribute themselves over a set of habitat patches, filling in the highest quality patches first and colonizing lower quality patches only when doing so is better than settling in a higher quality but now crowded patch.
The short, final portion of the small intestine, running from the jejunum to the large intestine.
ilium (plural, ilia)
One of the three paired bones fused to form the pelvic girdle; the ilium forms the cranial and lateral portion, and in birds is completely fused with the ischium and the synsacrum. It has a cup-shaped depression for the attachment of the femur (thigh bone).
immediate variety
Pattern of singing in which a bird sings a song type once, then moves on to a different song type and sings it once, then goes on to yet another type, and so on, such that the bird sings many different songs in its repertoire before ever repeating one.
The movement of individuals into a population.
immune system
The organs, tissues, cells, and antibodies that differentiate self from non-self and which act in concert to neutralize potentially pathogenic organisms or substances.
An individual bird’s ability to mount a functioning immune response after exposure to a pathogen or antigen.
The study of the immune system.
imperforate septum
A condition in which the nasal septum (tissue separating the left and right nasal cavities) has no opening. In contrast with a perforate septum (see separate entry), an imperforate septum appears to decrease an animal’s sensitivity in detecting odors. However, it increases the ability to locate an odor’s source, because odors entering one nostril do not mix with those from the other.
Important Bird Areas (IBA) Program
A program to identify sites (called IBAs) in each state that are particularly important to local populations of breeding, wintering, or migrating birds. The program gathers input from bird watchers and professional ornithologists, designates the sites it feels are most important, and then encourages public and private stewardship of the sites. Developed as an international effort by BirdLife International, the program is conducted in North America by the National Audubon Society.
A type of early learning in which a young animal quickly acquires specific information for certain experiences. For example, after hatching, Greylag Goose goslings rapidly learn to follow the first moving object they encounter, which is usually their parent.
inattentive periods
In avian biology, blocks of time spent off the nest during incubation.
The mating of closely related individuals within a population. The greater the degree of inbreeding, the greater the chance that deleterious genetic traits will occur in the population. See inbreeding depression.
inbreeding depression
The reduction in the average fitness of offspring born to parents that are closely related to each other, compared to the fitness of offspring born to unrelated parents. Inbreeding depression occurs because closely related parents share more genes, and thus their offspring are more likely to receive two copies (one from each parent) of alleles that cause deleterious traits or genetic diseases. For example, in humans, the genetic disease hemophilia was once common among some of the inbred royal families of Eurasia.
A mating between close relatives.
incest taboo
The behavioral avoidance of breeding with close relatives; it has undoubtedly evolved in many organisms because it reduces inbreeding depression (see separate entry). Because determining relatedness can be difficult, many species seem to avoid incest by not breeding with any individual with which they have been in close contact during their developmental period (which is usually a sibling or parent).
The process by which animals that lay external eggs keep those eggs at the proper temperature for embryonic development until they hatch (or the nest fails). Only birds, crocodiles, pythons, and monotremes (egg-laying mammals) incubate their eggs. In most cases, birds sit on their eggs to keep them warm, but many megapodes bury them—in piles of decaying vegetation, in long tunnels or broad pits where the earth is warmed from nearby hot streams or volcanic cinder fields, or in pits or burrows where bare sand or soil is heated by the sun. In very hot environments incubation may require cooling the eggs by shading them, burying them in sand, or keeping them moist.
incubation patch
See brood patch.
incubation period
The time from the start of regular, uninterrupted incubation to hatching.
incubation pouch
A type of brood patch, found on the breast of some albatrosses, that is a featherless cavity surrounded by thick feathers into which the single egg fits so snugly that it may remain inside even when the bird stands.
indeterminate layers
Bird species that will lay additional (replacement) eggs if one or more eggs are removed from the clutch during laying. Once they have begun incubation on a full clutch, however, they cease to lay replacement eggs if more are removed. For comparison, see determinate layers.
A numerical sequence or other representation that indicates the relative level, degree, or amount of something or some property. For example, an index of population density, or an index of health.
indicator species
A species that acts as a “canary in a coal mine,” because changes in its population density or distribution provide an early warning that its habitat is changing in some way—often through degradation by human activity. Birds are good indicators because they are relatively noticeable and easy to survey, because many bird species are high in the food chain and thus sensitive to the bioconcentration (see separate entry) of toxins, and because many species have narrow habitat requirements. Also called biological indicators.
Members, along with whydahs, of the African genus Vidua (members of this genus are called viduines). Viduines are colorful seed-eaters that are brood parasites on other members of their family, Estrildidae; they are known for the intricate patterns inside the mouths of their nestlings, which strikingly resemble those of the nestlings of their hosts.
indirect fitness
An individual’s indirect fitness is the portion of its genes that is transferred into the next generation (and beyond) that occurs as a result of that individual’s blood relatives producing offspring. Some of an individual’s genes are propagated when a relative breeds because the individual shares a percentage of its genes with that relative (the percentage depends on the relatedness), and the relative passes 50 percent of its genes (both those that it shares and those that it doesn’t) to its offspring. This contrasts with direct fitness, which is the portion of genes transferred through the production of an individual’s own offspring.
individual recognition
The ability of an animal to identify other specific individuals.
inertial navigation
Finding your way to the correct destination by keeping track of all the turns and accelerations you have taken along the way. For example, logging in your brain the turns and accelerations of an outward trip and then integrating them to compute a direct route home. Inertial navigation requires no outside reference points.
information center hypothesis
The idea that one benefit of colonial breeding is that individuals who have been unsuccessful at finding food might find better feeding grounds by watching at the colony for successful foragers (those who return with food), and then following them to good hunting spots.
The flattened, funnel-shaped opening of the oviduct. In birds, when the ovary releases an egg, the infundibulum moves up to the ovary and opens, “swallowing” the egg much like a snake swallows a rat.
innate behavior
See instinctive behavior.
innate rhythmicity
The ability to contract without being stimulated by the nervous system, as found in cardiac muscle cells.
inner ear
Complex structure inside the ear in the form of a membranous, fluid-filled sac (the membranous labyrinth) floating inside a bony, fluid-filled sac (the bony labyrinth). The inner ear consists of the semicircular canals enclosing their ducts and the vestibule enclosing the utriculus and the sacculus—all of which are concerned with balance— and the cochlea and its enclosed cochlear duct, which are concerned with hearing.
inner shell membrane
In avian biology, the membrane just inside an egg’s outer shell membrane; it is thinner and less coarse than the outer membrane. The inner shell membrane contains the albumen and adheres tightly to the white of a hard-boiled egg, making it difficult to peel.
inner vane
The vane located on the medial side of a wing or tail feather. On a wing feather, the inner vane is on the edge of the wing that trails in flight. In flying birds, the inner vane is wider than the outer vane, producing an asymmetry that aids in flight.
inner wing
The portion of the wing from the wrist to the shoulder; the secondary feathers are located on one section of the inner wing.
innominate arteries (left and right)
Large arteries that branch from the aortic arch and then soon branch again into the carotid arteries to the head and neck and the subclavian arteries to the front limbs (wings).
insect-net theory
A variation on the cursorial theory of the origin of avian flight, first proposed by John Ostrom in 1976. The insect-net theory suggests that dinosaurian bird ancestors first evolved feathers as thermal insulation, and then, as running dinosaurs clapped the feathered forelimbs together to catch insects, eventually the motion became flapping flight.
The transfer of sperm from the male into the genital tract of the female; in birds, sperm enter the cloaca.
The end (attachment site) of a skeletal muscle that moves the most during contraction.
inshore feeders
Birds (or other animals) that forage close to shore, fairly near their nesting areas; includes birds such as terns and many gulls.
insight learning
A modification of behavior that occurs by evaluation of a situation rather than as a result of previous experience with a particular problem.
instinctive behavior
A behavior that is triggered in full form, without any learning, the first time an individual responds to the releaser; also called innate behavior or instinct.
integumentary system
The skin and structures that are produced by the skin, such as (in birds) feathers, color pigments, scales, claws, beak, wattles, and comb.
intention movements
Movements that are either incomplete (such as the initial stages of a behavior) or that indicate what the performer is about to do. For example, a bird about to attack may crouch down and tense its muscles. Many intention movements have become exaggerated and incorporated into displays.
interactive playback experiments
Computer-controlled playback experiments in which researchers can change various aspects of songs played to territorial birds in the middle of a singing encounter, based on the subject bird’s singing behavior. For example, researchers might choose to play longer or shorter songs, or songs that match those of the subject, or they might respond more slowly or more quickly, or overlap the subject’s singing.
In ecology, a situation in which one individual actively prevents other individuals from obtaining a limited resource. It may do so through various behaviors, such as aggressive interactions that set up dominance hierarchies or territories.
interference competition
Competition among individuals or species in which they interact directly—often via aggression—over access to the same resources.
intermittent flight
A common flight style among small and medium sized birds that consists of flapping phases interspersed with flexed-wing bounds or extended wing glides.
international union for the conservation of nature (iucn)
International nongovernmental organization based in Switzerland and devoted to the conservation of species. In 1963, the IUCN drafted the original text of the Convention on International Trade in Endangered Species, also known as CITES (see separate entry), which was finally ratified in 1975.
interpupillary distance
The distance between the centers of an individual’s two pupils; important in fitting binoculars.
intersexual selection
A subset of sexual selection, one involving the reproductive success of individuals that depends on the actions of the opposite sex. In birds, intersexual selection often (but not always) involves mate choice by females who are selecting among potential male mates.
interspecific competition
Direct competition for limited resources among individuals of different species.
interspecific territoriality
Aggressive, territorial behavior directed by individuals of one species against members of other species.
intestinal lymph trunk
Lymph vessel that carries the products of fat digestion from lymph vessels coming from the small intestine to the thoracic duct. These digestive products are first picked up at the small intestine by lymph capillaries, which carry them to progressively larger lymph vessels running along the surface of blood vessels in the intestinal wall. These vessels eventually join to form the intestinal lymph trunk, which carries the products to one of the two thoracic ducts, which eventually enter the cranial vena cava. The reason for this circulatory pattern, which occurs in (at least) all birds and mammals, is not understood.
intrasexual selection
A subset of sexual selection, one involving the reproductive success of individuals that depends on competition with members of the same sex. In birds, intrasexual selection often (but not always) involves competition among males over access to females.
intraspecific competition
Competition (for food, territories, mates, and so on) that occurs among members of the same species.
inverse density dependence
A feedback loop in which populations at higher density tend to grow larger, whereas populations that shrink tend to become even smaller over time.
Small, arboreal songbirds that search leaves, often in dense foliage, for insects. The four species form the family Aegithinidae, endemic to the Oriental zoogeographic region.
iridescent colors
Structural colors, sometimes brilliant, that shimmer and glitter because they change in brightness as the angle of view changes. The colors are produced when light waves reflected off thin films (in birds, structural layers within flattened feather barbules) interfere with one another, as can be seen in soap bubbles and most hummingbirds.
The colored part of the eye surrounding the pupil; it is part of the eye’s choroid (middle) layer. The iris contains muscle fibers and controls the diameter of the pupil and thus the amount of light that enters the eye.
irruptive migration
Migratory movements that are irregular in time and space, depending upon factors other than a change of seasons, such as food availability. For example, the seeds and buds eaten by finches such as Pine Siskins and redpolls fluctuate in abundance not only seasonally but from year to year and from region to region, so in some years large numbers of the birds move out of northern forests to breed, yet in others they stay put.
ischium (plural, ischia)
One of the three paired bones fused to form the pelvic girdle; the ischium forms the caudal and lateral portion.
island endemics
Species that evolved on an island and are found only upon that island.
The region of the avian oviduct after the magnum and before the shell gland; the isthmus secretes the egg and shell membranes and fluid albumen around the fertilized ovum as the ovum travels down the oviduct.
See International Union for the Conservation of Nature.


The long middle portion of the small intestine, running from the duodenum to the ileum.
Birding term for a quick impression of a bird’s major features. Jizz harkens back to the “general impression of size and shape” (G. I. S. S.) that British observers used during World War II to distinguish between enemy and friendly aircraft.
A written record of field observations of birds.
jugal arch
Bony rod on each side of the upper jaw below the palatine (another set of bony rods). In birds, as the lower jaw opens it moves the quadrate, which pushes the palatine and jugal arch forward, pushing on the premaxillary bones to raise the upper jaw.
jugular veins (left and right)
Paired veins carrying blood from the head and neck region; they merge on each side with the subclavian and pectoral veins to form the caudal vena cavae, which return the blood to the heart.
The lower part of the throat of birds, just below the gular region.
juvenal plumage
Feather coat worn by juvenile birds after they have molted their natal down; it consists of the first true contour feathers.
A young bird.


A midventral ridge of bone that projects outward from the sternum and provides a site for the attachment of the large pectoral flight muscles. Large, flightless birds lack a keel, as do the tinamous. Also called a carina.
A hard protein that forms scales and claws and is the primary structural component of mature feathers. Avian keratin differs from the keratin of all other animals in its amino acid sequence.
A large aggregation of birds, usually hawks, that are spiraling upward in a thermal.
key innovation
A newly evolved trait that allows a lineage to take advantage of new ecological opportunities, and which results over time in that lineage diversifying into multiple species that are specialized for those new ecological niches.
keystone species
A species that has a disproportionately large effect on its habitat, biological community, or ecosystem. Named after the top block of stone—the keystone—in an archway that is essential for holding the other stones in place.
See kilohertz.
In avian biology, the aggressive takeover of a brood of young by adults that are not the parents.
Paired organs of the excretory system that are irregular in shape and, in birds, are each composed of three interconnecting lobes; they remove waste products from the blood, especially nitrogenous wastes, and form a highly concentrated urine that, in birds, is passed to the cloaca via the ureters. Kidneys also maintain a balance of salt ions in the blood.
kilohertz (khz)
A measure of the frequency of a sound, in thousands of cycles (of compression and thinning of the air) per second. One kilohertz equals 1,000 Hertz. Also written kiloHertz.
kin selection theory
A line of reasoning suggesting that the closer the kinship (degree of relatedness) between two animals of the same species, the greater will be their tendency to cooperate with one another in various ways. Such cooperation results from natural selection because, if an animal enhances the survival of a relative, it also enhances its own indirect fitness (see separate entry); the closer the relative, the greater the degree to which its own indirect fitness is enhanced.
Level of classification of organisms above “phylum”; similar phyla are placed within the same kingdom. All birds are in the kingdom Animalia.
A family (Apterygidae) of three species of grouse-sized, flightless, nocturnal ratites endemic to New Zealand; kiwis probe soil with their long beaks, using their keen sense of smell to locate earthworms.
A parasitic interaction in which one bird steals prey or food from the individual who originally obtained it.
See gastric cuticle.
k-selected species
Species in which individuals have a life history strategy that relies on longevity, rather than a high reproductive rate, to maximize the number of offspring produced during their lifetimes. Such species tend to be large, develop slowly, begin breeding at a relatively old age, have few young per cycle (small clutches or litters), breed infrequently, take care of their young for extended periods, and have long life spans.
k–t event
The massive, worldwide extinctions 65 million years ago at the end of the Cretaceous Period and the beginning of the Tertiary Period, which wiped out dinosaurs as well as many other plants and animals. The extinctions are thought to have resulted from worldwide climatic disturbance caused by a large meteor colliding with Earth.


lacrimal gland
One of the two tear glands on each side of the eye; the lacrimal gland lies in the lower part of the orbit of the eye and has many ducts entering the space between the lower lid and the cornea. Its secretions moisten the eye and nourish the cornea.
laminar flow
A smooth flow of air over an airfoil.
landbridge islands
Islands that were formerly part of the nearby mainland and which often retain some of the birds and other organisms from that earlier period. that and which were created by a rise in the level of the ocean. In contrast, oceanic islands rose directly out of the sea, and all land organisms on them had to have arrived by over-water colonization.
large intestine
Short, straight tube extending from the small intestine to (in birds) the cloaca; it holds the intestinal contents while water is being reabsorbed, and passes the remainder to the cloaca.
laryngeal cartilages
Two major cartilages that make up the sides and floor of the larynx. Also called the cricoid cartilages.
laryngeal folds
Fleshy folds in the lower surface of the pharynx that surround the glottis, the opening to the larynx.
Structure at the upper end of the trachea that consists of several cartilages; it acts as a valve, regulating the flow of air into the trachea. In mammals the larynx is the voice box, but in birds it produces no sound, leaving sound production to the syrinx.
Toward the side of the body; away from the midline.
lateral labia
Important sound-producing membranes in the lateral wall of each half of the syrinx. During sound production muscles move the bronchi upward into the trachea, twisting the third bronchial cartilages so that the lateral labia and medial labia (in the medial walls) move toward (and close to) each other and into the path of air flowing out of the respiratory system. Sound is produced as air rushes between the lateral and medial labia, causing these soft tissues to vibrate.
latitudinal diversity gradient
The general rule that in most groups of organisms, species diversity is greatest in the tropics and steadily decreases towards the earth’s polar regions.
The northern land mass formed 200 million years ago when Pangea split into two large land masses. It consisted of present-day North America, Europe, and Asia.
leaf tossing
A foraging technique in which a bird tosses aside the leaf litter (with its beak or feet) to search for food; used by towhees, turkeys, thrashers, and many sparrows.
Eight species of oriole-sized arboreal songbirds endemic to the Oriental zoogeographic region; leafbirds are mostly green and yellow, and feed mainly on insects and fruit. Together with fairy-bluebirds, they form the family Irenidae.
leapfrog migration
A pattern in which the individuals at the extreme of the breeding distribution migrate farthest, in the process traveling past birds that breed and winter in intermediate locations.
learned behavior
A behavior that requires some amount of previous experience—such as exposure to a stimulus—as well as a memory of that experience, before it is carried out fully.
The modification of a behavior as a result of experience.
leg spurs
Bony outgrowths near the distal end of the tarsometatarsus, covered by a pointed horny sheath; used as weapons by male chickens, peafowl, and many other pheasant relatives.
A traditional courtship area where many males of the same species gather to attract females for mating; each male spends a large amount of time defending a small site at which he displays to compete with other males and, in particular, to earn copulations with sexually receptive females who visit the lek to choose among the males. In most species, the top few males secure most of the matings. The lek contains no nest sites, food, or other resources useful to nesting females.
lek paradox
A theoretical conundrum in a population in which all females mate only with the rare highest-quality males, all offspring will inherit the traits that make those males so attractive, and hence over a few generations there will be little genetic variation among males in those important traits.
lek polygyny
Mating system in which males gather in leks and display to earn copulations with visiting females. In most lekking species, the top few males secure most of the matings, resulting in a high degree of polygyny.
Spherical or ovoid structure near the front of the eye; it changes its curvature to sharply focus images from varying distances on the retina. The lens is crystalline-like and composed of regularly oriented layers of collagen fibers.
One of the two major groups of diapsid reptiles; it contains all snakes and lizards as well as the ancient ichthyosaurs and plesiosaurs.
lesser coverts
The feathers on the upper surface of the wing that partly overly the median coverts and extend to the marginal coverts.
See white blood cells. Also spelled leucocytes.
Lévy flight
A pattern of movement based on mathematical models that generate a mix of short random movements and occasional longer movements.
A woody vine.
life history theory
A set of ideas that attempt to explain the diversity in the breeding strategies of living things by looking at why various characteristics relating to an organism’s birth, death, and reproduction have evolved. These characteristics, called life history traits (see separate entry), include such things as number of offspring, age at first breeding, and length of the developmental period.
life history traits
Characteristics of living things that are related to birth, death, and reproduction. Examples include the number of offspring, the age at first breeding, the interval between breeding cycles, and the chance of surviving to various ages. Collectively, these characteristics make up an organism’s life history or life history strategy.
life list
Record of every species seen by a particular person, noting the date and location of the first sighting. People may keep life lists of different groups of organisms, such as birds, butterflies, or reptiles.
life table
An actuarial table that describes the probability of death at each age, and which sometimes also includes the expected reproductive output at each age.
life zone
A geographic region (or, in mountains, an elevational band) characterized by a distinct group of animal and plant species.
Force acting on a moving airfoil (such as a bird’s wing) perpendicular to the direction of airflow; in a bird that is gliding or flying horizontally, lift acts upward in opposition to gravity.
lift-to-drag ratio
The lift produced by a wing divided by the drag it experiences while flying. Long, narrow wings have the greatest lift-to-drag ratios and are the most efficient.
Fibrous connective tissue that connects one bone to another across a joint.
limiting factor
Something that is in (or missing from) a particular environment and that prevents a particular species from living or breeding in that place. Examples of limiting factors include a large number of predators and a low level of a critical resource, such as food.
Extinct, flying, chicken-like palaeognathous birds thought to have been common in North America and Europe during the early Tertiary; they may be the ancestral stock that gave rise to ratites all over the world.
The largest internal organ in the body; in birds the liver has two lobes and performs a variety of functions, including secreting bile to help emulsify fats for digestion, storing sugars and fats, forming uric acid, and removing foreign substances from the blood.
local movements
The non-migratory movements involved in the daily activities of birds.
local population
A population confined to a small area; for example, all the Blue Jays in one neighborhood.
Small area between the eye and the base of the upper beak.
low pressure system
A weather system consisting of cold and warm air masses circulating around an area of low barometric pressure. In the Northern Hemisphere the masses circulate counterclockwise; in the Southern Hemisphere they move clockwise.
lower beak
The lower half of the beak; also called the mandible or lower jaw.
lower critical temperature
The environmental temperature below which the body of a bird or mammal increases its metabolic rate and employs physiological responses to warm itself, assuming behavioral responses are no longer adequate.
lower jaw
See lower beak.
lower lethal temperature
The environmental temperature below which a bird or mammal cannot keep its body warm enough to survive.
lumbar vertebrae
The vertebrae of the lower back; in birds they are all fused with the sacral vertebrae and some of the thoracic and caudal vertebrae to form the synsacrum.
lumbosacral enlargement
Swelling along the spinal cord at the level of the legs; it is associated with the lumbosacral plexus.
lumbosacral plexus
Plexus (see separate entry) along the spinal cord at the level of the hind limbs; it is associated with the lumbosacral enlargement of the spinal cord.
Informal term for changes in classification that involve merging into one species two or more taxa that were previously each considered to be a separate species.
Fluid carried in lymph vessels; also see lymphatic capillaries and lymphatic system.
lymphatic capillaries
Tiny vessels that form a network intertwining with the capillary beds of the arterial and venous systems throughout most body tissues. Some of the tissue fluid that has lost its oxygen and nutrients to the body (tissue) cells and picked up carbon dioxide and other wastes diffuses into the lymphatic capillaries, and is then termed lymph. The lymphatic capillaries lead to progressively larger lymphatic vessels, which eventually dump their contents into the venous system.
lymphatic system
Organ system composed of lymphatic vessels, ducts, and nodes that gathers tissue fluid that has leaked from the blood capillaries, filters foreign substances and old or damaged cells from the fluid, and then returns the fluid to the general blood circulation. The lymphatic system also releases antibodies and transports the products of fat digestion from the intestines to the venous system, bypassing the liver.
Two pheasant-sized songbirds (forming the family Menuridae) of Australian rain forests, named for the elaborate, harp-shaped tail of the Superb Lyrebird. They have loud songs and calls and are fantastic mimics, sometimes clearly reproducing mechanical sounds, such as logging trucks and chain saws, along with the songs of other birds.


The evolution of new species over long periods of time, such as thousands of years (see microevolution).
magnetic anomaly
A place where the earth’s magnetic field is disturbed, usually by large deposits of iron near the surface.
magnetic compass
A mechanism by which birds and some other animals can use the earth’s magnetic field to determine compass direction.
magnetic field of the earth
The region around the earth in which objects experience magnetic force (which is a vector—a quantity with both strength and direction). The field is created by currents generated by the constant motion of molten iron in the earth’s core. In effect, the earth is a large magnet with two magnetic poles (near the North and South Pole). The field is stronger closer to the magnetic poles and weaker toward the equator.
magnetic map
A mechanism by which birds (and some other animals) may be able to use the earth’s magnetic field as a map. For example, because the magnetic field is strongest toward the poles, a bird might use the strength of the field at any given point to estimate latitude.
The first region of the avian oviduct, after the infundibulum; the magnum is glandular and secretes the first of the albumen or “white” of the egg around the fertilized ovum as it travels down the oviduct.
malar region
Small area caudal to the base of the lower beak; also called the cheek.
malar stripe
A distinctively colored stripe in the malar region of birds; also called a mustache stripe or whisker stripe.
A Neotropical suboscine family (Pipridae, 44 species) of small, frugivorous birds noted for the stereotyped courtship displays of the colorful males. Many manakin species form leks, and most displays involve intricate maneuvers that may be enhanced by mechanical and/or vocal sounds and plumage displays.
In birds, the lower half of the beak, called the lower beak or lower jaw.
mandibular nerve
A division of the trigeminal nerve (the fifth cranial nerve) after it leaves the brain; the mandibular nerve carries sensory input from the lower beak and corner of the mouth, and motor output from the brain to the muscles of the lower beak.
A group that includes birds and their closest relatives within the theropod dinosaurs.
The portion of the forelimb distal to the wrist; also called the hand. The primary feathers attach to the manus.
map and compass model
The theory (proposed by Gustav Kramer in 1953) that two things are required to navigate to a particular destination from an unfamiliar location
marginal coverts
The feathers covering the upper surface of the wing, from the leading edge back to the greater coverts, which they partly overlie.
marine birds
See seabirds.
market hunting
Intensive hunting of species to obtain meat, feathers, hides, or other body parts highly prized by commercial consumers.
mass extinction
In earth history, the extinction of an exceptionally large proportion of species within a relatively short period of time.
matched countersinging
Interaction in which two birds sing back and forth to one another, each choosing songs that are identical or similar to those of the other, or that contain similar phrases. Not well understood, but thought to be one way in which birds “duel” through song.
mate guarding
A behavior in which a male attends closely to his mate during her fertile period to help ensure that she does not mate with other males.
mating system
The pattern in which matings are distributed among males and females within a population, sometimes but not always involving longer-term social bonds.
matrilineal societies
Groups of animals in which the females remain in their natal colonies but the males disperse when they become adults. Thus, within a colony, the females share more genes than do the males, and are more likely than males to engage in acts that appear altruistic toward one another.
In birds, refers to the upper half of the beak; the upper jaw.
maxillary nerve
A division of the trigeminal nerve (the fifth cranial nerve) as it leaves the brain. It carries sensory input from the skin of the face, upper jaw, upper eyelid, and conjunctiva (tissue covering the eye).
maximum range speed
The flight speed that maximizes the distance traveled for a given amount of energy.
mechanical isolation
A barrier to interbreeding between two populations that arises from an incompatibility between the male genitalia of one species and the female genitalia of the other species.
Toward the midline of the body.
medial labia
Important sound-producing membranes in the medial wall of each half of the syrinx. During sound production muscles move the bronchi upward into the trachea, pushing the medial labia and lateral labia (in the lateral walls) toward (and close to) each other and into the path of air flowing out of the respiratory system. Sound is produced as air rushes between the lateral and medial labia, causing these soft tissues to vibrate.
On the midline of the body.
median coverts
The covert feathers lying between the lesser and greater coverts on the upper surface of the wing.
medulla oblongata
The most posterior portion of the brain, also called the brain stem, where the nuclei of most of the cranial nerves are located. The medulla oblongata extends caudally through the foramen magnum to become the spinal cord.
Continental assemblages of exceptionally large-bodied mammals and birds, most of which (except in Africa) became extinct as humans spread across the world’s continents and islands.
A family (Megapodiidae, 21 species) of chicken-like terrestrial birds of the Australasian and Oriental regions. Megapodes do not use their own body heat to incubate their eggs; instead, in many species the males tend the eggs of several females in huge, warm mounds of decaying vegetation. Other species use geothermal heat to warm their eggs.
Pigment, usually present as tiny granules, that produces a range of earthy colors from dark black, brown, and red-brown to gray, yellow-brown and pale yellow. Birds can synthesize their own melanin by oxidizing the amino acid tyrosine.
membranous labyrinth
A system of interconnected, fluid-filled canals (containing endolymph) floating in the bony labyrinth of the inner ear. The labyrinth forms the cochlear duct, the semicircular ducts, and the utriculus and sacculus.
General term for the vascularized membranes surrounding the brain and spinal cord. Specifically, these are the outer fibrous dura and the inner arachnoid and pia layers. Meninges provide sustenance and waste removal for the cells of the brain and spinal cord, which are not served by the circulatory system.
See midbrain.
A family (Mesitornithidae, 3 species) of rail-like, ground-dwelling birds endemic to Madagascar; mesites are about the size of a Mourning Dove.
The continuation of each bronchus (right and left) as it enters the lung and loses its cartilaginous half-rings; also called the primary bronchus. The mesobronchi gradually decrease in diameter and branch into secondary bronchi. Only in birds is the term “mesobronchus” used as an alternate name for the primary bronchus.
metabolic rate
The rate of energy production (often measured as heat production) in an organism.
metabolic water
Water obtained by an organism’s body as a byproduct of the chemical breakdown of fats, carbohydrates, and proteins within the body.
All the chemical processes that take place in the cells and tissues of the body.
Palm bones. In humans they remain distinct, but in birds, they are fused with some of the carpals (wrist bones) to form the large carpometacarpus.
All the individuals of a species living in an area that contains a set of subpopulations (local populations) close enough together so that individuals from occupied habitat patches can disperse and recolonize patches where the species has gone extinct.
metapopulation dynamics
The changes in local populations found within a metapopulation over time, in which the local populations (in a habitat patch) may go extinct and then individuals from within the metapopulation may recolonize the patch at a later date, producing a long-term situation in which local populations “wink” out and then “wink” back on.
metapopulation models
See population viability analysis.
Instep bones. In humans they remain distinct, but in birds they fuse with the distal tarsals (ankle bones) to form the tarsometatarsus, the long bone supporting the upper section of the foot.
See tarsometatarsus.
A change in the frequency of a genetically controlled characteristic in a population over a relatively short period of time (see macroevolution).
The middle region of the vertebrate brain; it contains the optic lobes (which in birds are huge and dominant), as well as regions for the input and processing of information on hearing and balance. Also called the mesencephalon.
middle ear
Air-filled chamber between the eardrum and the cochlea (inner ear); in birds it houses the columella.
mid-story vegetation
Layer of vegetation in a forest that consists of trees whose crowns are below the level of the main canopy and above the understory vegetation (see separate entry).
The regular movement of all or part of a population to and from an area; usually refers to seasonal journeys to and from breeding grounds or feeding areas.
migratory connectivity
The degree to which individuals breeding in a particular region migrate to the same over-wintering region.
migratory divides
Locations where two populations of the same species with different migratory behaviors meet and potentially interbreed.
migratory fallouts
Situations in which very large numbers of migrating birds are forced to land in a small area by adverse winds or other challenging weather conditions.
migratory program
The genetically programmed information that guides an inexperienced bird on its first migration. The timing of that migration is controlled by an internal biological clock that controls circannual rhythms, and the bird chooses the approximate direction and distance through its built-in ability to carry out vector navigation.
migratory restlessness
Nocturnal hopping and fluttering during the normal migratory period, performed by caged birds that normally are inactive at night. Researchers often use the degree of this “unrest” as an indication of a bird’s desire to migrate, and the orientation of the hopping and fluttering to indicate the compass direction in which the bird would normally be migrating. First called Zugunruhe by the German scientists who discovered it.
A situation in which one individual or species has evolved or learned to be similar to another in appearance, behavior, or sound. For example, Brown Thrashers and Northern Mockingbirds imitate the songs of other bird species.
mineral lick
General term for a spot that animals visit to obtain minerals, either naturally occurring in the soil or provided by humans. A clay lick (see separate entry) is one type of mineral lick.
minimum power speed
The flight speed that maximizes time in the air for a given amount of energy.
minimum viable population
The smallest population size considered likely to allow the species (or population) to avoid extinction for a specified number of years.
mitigation bank
See wetland mitigation.
Membrane-bounded units in all cells except bacteria; they generate most of the cell’s energy. Mitochondria contain DNA in the form of a ring.
mitochondrial dna (or mtdna)
In birds, the DNA that is found not in the cell nucleus, but rather in the cell’s mitochondria, a structure involved in generating energy within the cell. Mitochondrial DNA is passed down only from mother birds to their offspring.
mixed-species flocks
Flocks of birds that contain individuals of more than one species.
mnemonic device
A memory aid. In birding, usually refers to the association of phrases from human speech with the songs of particular birds, to help people remember the songs more easily.
A family (Dinornithidae) of at least 22 species of enormous, flightless ratites that roamed the open foothills and tussock lands of interior New Zealand. Moas evolved in the late Tertiary, but were extirpated by the Polynesians approximately 400 years ago. The largest moa stood 14 feet tall.
Behavior in which a number of birds (often different species) swoop and dash at a potential predator; they usually give broad-band, raspy calls (mobbing calls) that are easy to locate and thus attract additional birds.
model song
A song that a bird listens to and attempts to duplicate during the process of song learning.
molecular clock
The idea that DNA changes accumulate at a relatively steady rate, and that the genetic differences among organisms can therefore be used to estimate the time in the past when they started to diverge from one another.
The process of shedding all or part of the feather coat and replacing it with new growth.
monocular vision
Type of vision that produces flat, two-dimensional images, in contrast to binocular vision, which produces three-dimensional images. Monocular vision results when the eyes are positioned on the sides of the head such that an object can be seen by one eye or the other, but not by both eyes at the same time.
Mating system in which one male pairs with one female, at least for a given breeding season.
Describes species in which males and females are similar in outward appearance.
monophyletic group
In evolutionary terms, any group of organisms that includes the most recent common ancestor of that group and all of its past and present descendants.
A set of individuals within a species that are similar to one another in some genetically determined morphological characteristic, but are distinctly different from other sets of individuals within that species. Morphs may differ in characteristics such as color, body size, or bill length or shape, but not in characteristics that are related to sex, age, locality, or season.
mortality rate
See death rate.
motion parallax
A depth cue that results from the motion of the observer, as objects that are closer move farther across the visual field than do objects that are more distant.
motor neurons
Nerve cells that convey impulses from the brain and spinal cord to stimulate a muscle to contract or permit it to relax, or to cause a gland to secrete.
mound nest
A nest composed of a pile of material with an egg chamber in the middle; in some species the chamber is entered through a tunnel from the outside. Mound nests are built by many megapodes, which bury their eggs in a pile of decomposing organic material; the Hamerkop of Africa, which builds a massive mound of sticks; and Monk Parakeets and Palmchats, which build large, colonial mound nests.
Long-tailed African birds (family Coliidae, 6 species), about the size of a Mourning Dove, that climb through vegetation using their stout, hooked beaks; frequently mousebirds perch in a “chin-up position,” hanging vertically from a branch. Also known as colies.
mudnest builders
A family (Grallinidae) of two striking, black-and-white, robin-sized birds named for their large, cup-shaped, mud nests. The Magpie-lark is widespread, abundant, and well-known throughout open areas in much of Australia; the Torrent-lark inhabits fast-flowing streams in the mountainous areas of New Guinea.
muscle fibers
Bundles of long, cylindrical, contractile cells (those that can contract) that are the functional units of muscle tissue; they shorten when stimulated by a nerve impulse.
mustache stripe
A distinctively colored stripe in the malar region of birds; also called a malar stripe or whisker stripe.
A change in the sequence of nitrogenous bases in DNA. Most mutations have little or no effect on an organism, but a few result in changes in the structure or type of proteins produced, thus creating the genetic diversity upon which natural selection can act.
mutual displays
Intricate, synchronized displays or dances performed by members of a mated pair; the displays appear to stimulate and coordinate breeding behavior between pair members and to reaffirm the pair bond. Given by many long-lived birds that mate for life, such as albatrosses, gannets, grebes, and penguins.
An association between two (or more) organisms in which both (or all) organisms benefit.
mycoplasmal conjunctivitis
A highly contagious infection of the conjunctiva of the eyes that has spread rapidly through House Finch populations in eastern North America since its discovery in Maryland in 1994. The disease also affects Purple Finches, American Goldfinches, Evening Grosbeaks, and Pine Grosbeaks, although to a far lesser extent. Many birds that contract the disease eventually die—usually from starvation, predation, exposure to the elements, or some other factor that results from having impaired eyesight. The disease is caused by a previously unknown strain of the bacterium Mycoplasma gallisepticum; other strains had long been known in poultry. Also called House Finch eye disease.
Fatty material forming the nerve sheath around many axons. The process by which myelin is deposited in a developing embryo and young animal is called myelination.
myelin sheath
The pale, fatty wrapping that surrounds the axons of many neurons; it insulates the axon, functioning, in part, like the insulation around household wiring.
See myelin.
A red-pigmented protein found in muscle cells. Similar in both form and function to the hemoglobin in blood cells, myoglobin binds with oxygen, storing it until the muscle cell needs it to release energy.
Visual condition of an individual in which distant objects blur because images are focussed in front of, rather than on, the retina; also called nearsightedness. Some types of birds, such as penguins, are myopic on land because their eye is designed for vision under water.


The back of the neck.
nares (singular, naris)
The openings of the nasal cavity; they are located in the upper beak, usually near its base. Also called nostrils.
nasal region
Portion of the skull containing the nostrils.
nasal-frontal hinge
See craniofacial hinge.
natal dispersal
The movement of a young animal away from the area in which it was born and raised. In birds, natal dispersal usually occurs between fledging and the first breeding season.
natal down
The soft down feathers covering young birds before they molt into juvenal plumage.
natural Selection
An evolutionary process in which the higher survival and reproduction of individuals with favorable inherited traits causes those traits to increase in the population over time.
The ability to determine an absolute geographic location in two dimensions.
Nearctic region
Zoogeographical region including arctic, temperate, and subtropical North America, reaching south to the northern border of tropical rain forest in Mexico. In the Home Study Course Greenland is included in the Nearctic, but some sources may place it in the Palearctic.
neighbor recognition
In avian biology, the ability of territorial birds to identify their neighbors—usually on the basis of song alone.
One of the two Superorders of ornithurine birds; it contains all birds with a neognathous palate—one in which the vomer and basipterygoid process are reduced and a flexible joint exists between the pterygoid and palatine bones (as compared to a palaeognathous palate, which is formed by larger, more rigid bones). Neognathae includes all modern birds except ratites, as well as the extinct Diatryma and dromornithids.
See Neognathae.
One of the two groups of living birds, containing all living birds except the ratites.
neomorphine cuckoo
See cuckoo.
An evolutionary phenomenon in which juvenile traits are retained into adulthood.
Neotropical migrants
Birds that winter in the Neotropics but migrate to the Nearctic region to breed. Examples include many wood-warblers, tanagers, and orioles.
Neotropical region
Zoogeographical region including the West Indies, South America, and Central America north to the northern edge of the tropical forests in Mexico. Also called the Neotropics.
Structures within the kidney that filter wastes from the blood to produce urine.
A bundle of many nerve cell fibers, surrounded and bound together by connective tissue; one nerve is large enough to be seen with the naked eye.
nerve cell
See neuron.
In avian biology, a structure built, excavated, or taken over by a bird, in which the eggs are laid and remain until they hatch. In many species, the young remain in the nest until they are able to fly. In some species, the “nest” is simply a scrape or depression on the ground. See specific nest types, such as cup nest, for more information.
nest appropriation
Nesting in a nest that previously was used by another species or another member of the same species—usually after the previous breeding attempt has ended. Nest appropriators include cavity adopters as well as species that take over open nests, such as Solitary Sandpipers, Bonaparte’s Gulls, House Sparrows, Great Horned Owls, Little Swifts, and many others.
net primary productivity
See primary productivity.
Cells that form a supporting, protective, felt-like bed for neurons and also provide electrical insulation. Not nerve cells themselves, they also are called glia or glial cells.
A nerve cell. The basic unit of the nervous system, it consists of a cell body, axon, and one or more dendrites. It is capable of generating, conducting, and receiving nerve impulses.
New World cuckoo
See cuckoo.
New World warblers
A family (Parulidae) of 115 species of small, insectivorous birds, many of which are colorful, found in the New World. Many are Neotropical migrants. Also called wood-warblers.
New Zealand Wrens
A family (Acanthisittidae) of tiny (smaller than a kinglet), nearly tailless, primarily insectivorous suboscine birds endemic to New Zealand. One of the four known species, the Stephens Island Wren, became extinct in the late 1800s.
See nongovernmental organization.
The role played by a particular species in its environment. Niche includes the many ways in which a species interacts with its physical and biotic environment, such as what and how a species eats, what temperatures it requires, where it spends time, when it is active, whether it disperses seeds or pollinates plants (if an animal), what animals pollinate it (if a plant), what preys on it, and so on. Also called ecological niche.
niche axes
Particular components of the environment that influence whether a species is present or absent, or how it interacts with members of the same or other species.
niche breadth
The range of resources utilized by a species; a species with wide niche breadth is referred to as an ecological generalist, whereas a species with narrow niche breadth is termed an ecological specialist.
niche overlap
The extent to which two species use the same resources or occur in the same habitats or locations.
niche shifting
Describes a situation in which a species occupies a different niche in different communities, depending on which competitors are found in each.
nictitating membrane
A thin, translucent fold of skin that sweeps sideways across the eye from front to back, moistening and cleaning the eye and protecting its surface. Found in many vertebrates, including birds, but not in humans.
The condition of chicks that stay in the nest after hatching.
The condition of chicks that leave the nest soon after hatching.
night flight calls
Calls given by migrating birds as they fly at night.
nnate immunity
Components of the immune system that are standard responses to a potential pathogen, not arising from or requiring previous exposure to that pathogen.
Active at night.
A clump or mass of tissue, for example lymph nodes
The points on a phylogeny where branches separate, usually representing a point where an ancestral lineages splits into two more more descendent lineages.
Pattern of movement in which individuals are constantly on the move, showing no tendency to return to previously occupied places. Crossbills and perhaps Budgerigars may be considered nomadic.
nonbreeding plumage
See basic plumage.
nongovernmental organization (NGO)
A nonprofit (not-for-profit) organization.
noniridescent structural colors
Colors produced in birds when tiny vacuoles (pockets) of air within cells in the feather barbs scatter incoming light. All blues and whites of birds are noniridescent structural colors, as are many greens.
North American Waterfowl Management Plan
A 1986 agreement between the United States and Canada to cooperatively protect waterfowl by jointly protecting habitat, restoring declining species, and conducting population research. Each year biologists make detailed population estimates, and then use them to help regulate the number of individuals of each waterfowl species harvested during the waterfowl hunting season.
Winter storm that sweeps into New England across the Atlantic Ocean from the northeast, the high winds circulating around a low pressure area. Often northeasters push pelagic birds from the open ocean near or over land; in severe storms, many birds die. Also called Nor’easter.
Northern Marine Region
The major faunal region of the seas that includes the frigid waters of the Arctic south to about 35 degrees north latitude.
northern timberline
See arctic tundra/coniferous forest ecotone.
notched tail
See emarginate.
nuclear species
Refers to the one or more species within a mixed species flock that are central to its formation and cohesiveness.
The basic molecular building blocks of DNA and RNA.
nucleus (plural, nuclei)
The membrane-bounded command center of all cells except bacteria; usually the nucleus contains the chromosomes.
nuptial plumage
See alternate plumage.


objective lens
The large lens in a pair of binoculars or a telescope that is farthest from the eye; it receives the image viewed in the eyepiece.
obligate brood parasites
Bird species that always lay their eggs in the nests of other species, leaving the resulting young to be raised entirely by the host parents. For comparison, see brood parasite.
obligate migration
A pattern in which all individuals in a population are migratory.
obligate partial migrant
See partial migration.
obligatory annual migration
A type of migration in which all individuals of a species migrate to and from a particular area each year. Usually obligatory migration occurs in species whose breeding-area resources vary greatly from season to season in a predictable way—for example, in birds that eat insects or nectar and that nest at high latitudes where both insects and nectar become scarce or absent during winter.
occipital condyle
A prominent “bump” or peg on the base of the skull, with which the atlas (the first cervical vertebra) articulates. Birds have one occipital condyle, but mammals have two.
oculomotor nerve
The third cranial nerve. It controls eye movement by carrying motor output to the eye muscles; it also carries motor signals to the eyelid muscles and the tear gland of the nictitating membrane.
offshore feeders
Birds (or other animals) that hunt schooling fish far out to sea and far from their nests; includes most pelagic species such as albatrosses and gannets. Avian offshore feeders are also called pelagic feeders.
oil gland
A gland, located at the base of the tail on the dorsal side of the bird’s body, that secretes oils that birds spread over their feathers during preening. The oils keep the skin supple and the feathers and scales from becoming brittle, but they do not appear to waterproof the feathers. Also called the uropygial gland or preen gland.
A large, nocturnal frugivore of South America, related to nighthawks; the Oilbird is the only member of its family, Steatornithidae. Oilbirds use echolocation to reach their nests, which are located deep within caves.
Old World cuckoo
See cuckoo.
old-growth forests
Virgin (uncut) forests or forests that have remained uncut for a very long time and thus contain trees ranging from hundreds to thousands of years in age.
The sense of smell.
Relating to the sense of smell.
olfactory epithelium
The lining or surface tissue of the nasal cavities; it contains the sensory endings of the olfactory nerves, which carry input about odors to the brain.
olfactory lobes
Two lobes at the anterior ends of the cerebral hemispheres of the brain that are concerned with the sense of smell. They are relatively small in birds.
olfactory map hypothesis
The idea that homing pigeons may learn a gradient odor map of the vicinity of their home loft by associating airborne odors with the directions from which winds carry them past the loft. This gradient odor map would be based on small but systematic changes in the intensity or composition of odors over a large area; as a bird moves in a given direction, particular odors becomes steadily stronger or weaker.
olfactory nerve
The first cranial nerve; it carries the sensations of smell from the lining of the nasal cavity to the olfactory bulb of the brain.
Organisms that eat both plants and animals.
The study of birds’ eggs.
open-ended learners
Songbirds that remain able to learn new songs throughout all or most of their lives.
operational sex ratio
The ratio of fertilizable females to sexually active males in a given population. Thus, the ratio measures not just the breeders, but all individuals physiologically and behaviorally capable of breeding.
A flap partially covering the nares; it may help to keep out debris. Found in some ground-feeding birds such as starlings, pigeons, and domestic chickens.
ophthalmic nerve
A division of the trigeminal nerve (the fifth cranial nerve) as it leaves the brain; the ophthalmic carries sensory input from the nasal cavity (for nonolfactory nasal sensations), eyeball (for nonvisual eye sensations), upper eyelid, forehead, and upper beak. In ducks and geese, it carries sensory input from the bill tip organ.
opposite birds
A group of small to medium birds that lived in the Cretaceous period between 65 and 140 million years ago. They are called “opposite” birds because their metatarsals (the instep bones of humans) fuse to form part of the tarsometatarsus from the proximal end to the distal end, a direction opposite to that of modern birds. Also called enantiornithines.
optic chiasma
Site at which the optic tract coming from the left eye crosses the tract coming from the right eye just before entering the optic lobes of the brain.
optic lobes
Two large lobes of the midbrain; in birds, they dominate the midbrain and are proportionally larger than those of mammals. The optic lobes receive the optic tracts from the eyes and are the sites of much initial processing of visual information.
optic nerve
See optic tract.
optic tract
Bundle of sensory nerves carrying visual sensations from the retina of the eye to the optic lobe on the opposite side of the brain. This nerve cable is considered the second cranial nerve, although it is really a tract.
Optimal Foraging Theory
Models of foraging behavior in which it is assumed that animals maximize their food intake while minimizing the overall costs of obtaining it such as energy and time devoted to searching, capturing, and processing.
oral flanges
Brightly colored enlargements around the base of the bill in nestlings of many species in which the parents feed the young. The flanges extend from the corner of the mouth and taper toward the tip of the bill, and are well-supplied with tactile nerve endings. Touching a flange causes the mouth to spring open, and the colors may help parents to place the food properly.
Cavity in the skull that houses the eye. The eyes of most birds are so large that the left and right orbits nearly meet at the midline of the skull.
Level of classification of organisms above “family” and below “class”; similar families are placed in the same order. The scientific names of bird orders end in “iformes” (for example, Passeriformes).
A group of tissues, often of distinctive character and function, that aggregate to form a discrete structure with a particular function, such as the heart, stomach, or lung.
organ system
A group of organs whose various functions are coordinated to accomplish one or more of the basic functions of life, such as the digestive system or the respiratory system.
Oriental region
Zoogeographic region including all of Asia south and east of the Himalayan Mountains (India and Southeast Asia), as well as southern China and the islands of Indonesia and the Philippines, east to include the islands of Timor and Sulawesi.
The ability to determine bearings and directionality.
The end (attachment site) of a skeletal muscle that moves the least during contraction.
Any physical trait that has been exaggerated for social display by sexual selection; usually refers to conspicuous traits involved in mate attraction.
ornithischian dinosaurs
One of the two major groups of dinosaurs, called “ornith”-ischian (bird-hipped) dinosaurs because their hips superficially resembled those of modern birds. They were highly specialized herbivores.
Early flying machines whose crude wings were lifted by humans flapping their arms; they never got off the ground.
One of the two major subclasses of birds; it includes all modern birds as well as the Lithornithids, Ambiortiformes, Hesperornithiformes, and Ichthyornithiformes.
Describes birds in the Subclass Ornithurae.
A visual representation of a sound, plotted on a graph as relative loudness (vertical axis) versus time (horizontal axis). The vertical axis actually is a measure of the increase or decrease in air pressure (measured in micropascals) associated with the sound wave, which determines the loudness. The oscillogram does not show sound frequency.
oscine passerines
The “true songbirds,” one of the two groups within the order Passeriformes, the most diverse avian orders.
Members of Suborder Passeri, which is one of the two large suborders of Order Passeriformes (perching birds); oscines also are known as songbirds or true songbirds. Oscines have particularly complex voice boxes, which allow them to sing more complex songs than other birds.
Movement of water or another solvent through a semipermeable membrane into a region of higher solute concentration, tending thereby to equalize the concentrations of solutes between the two sides of the membrane.
Hardening or calcification of soft tissue (such as cartilage or tendon) into bone or a bone-like material.
The largest living bird and the only member of its order and family, Struthionidae. This familiar, flightless ratite of African deserts and savannas is almost entirely herbivorous. Ostriches breed communally, with several females laying eggs in the same nest.
Of or relating to the ear.
outbreeding depression
The reduction in the degree to which local populations are adapted to their local environments as a result of individuals dispersing and mating with individuals in other populations (outbreeding). The reduction occurs because outbreeding individuals bring alleles adapted to one local environment into another, where they may be less advantageous.
outer shell membrane
A loose, fibrous membrane that lines the inner surface of the avian eggshell; it sticks tightly to the eggshell, helping to hold it together.
outer vane
The vane located on the side of a wing or tail feather that is away from the midline of the bird. On a wing feather, the outer vane is on the edge of the wing that leads in flight. In flying birds, the outer vane is narrower than the inner vane, producing an asymmetry that aids in flight.
outer wing
The portion of the wing from the wrist to the wing tip; the primary feathers are located on the outer wing.
ova (singular, ovum)
Female reproductive cells, also called egg cells or eggs, both before and just after they are fertilized by a sperm cell.
Describes an egg that is shaped like a chicken egg; also called ovate.
oval window
Former name for the vestibular window.
The female gonad; it matures and releases egg cells (ova) periodically throughout the breeding season in a process called ovulation, and also produces the sex hormones estrogen, progesterone, and testosterone. In most birds only the left ovary is functional, and it enlarges greatly during the breeding season.
See oval.
A wood-warbler (family Parulidae) that breeds in mature deciduous forests of the eastern and central United States and migrates to the Neotropics for the winter.
A diverse Neotropical suboscine family (Furnariidae, 240 species) whose members are especially numerous in temperate South America; ovenbirds are named for the oven-shaped, clay nests built by some species.
overstory vegetation
The mature trees that make up the canopy of a forest.
The tube that transports the egg from the ovary to (in birds) the cloaca; it is suspended from the dorsal body wall by a curtain-like membrane. After the egg enters the oviduct as an egg cell (ovum) and is fertilized, different sections of the ovary add different substances to the ovum to produce the hard-shelled egg. In birds, only the left oviduct is functional.
The process by which a substance is chemically combined with oxygen; also called “burning.” When body cells combine oxygen with the products of food digestion, such as carbohydrates, the reaction releases energy and two waste products—water and carbon dioxide.
Two bluebird-sized species in the starling family (Sturnidae) that climb upon large grazing mammals in the African savannas, removing ticks, insects, and the scabs of skin wounds—a behavior that benefits both the birds and their hosts. Also called tickbirds.
oxygenated blood
Blood whose red blood cells contain high levels of oxygen. Also called oxygen-rich blood, it is found in all arteries except the pulmonary artery and its branches.
oxygen-poor blood
See deoxygenated blood.
oxygen-rich blood
See oxygenated blood.


The social bond between a mated pair (usually a male and female) in a socially monogamous species, usually lasting for one breeding attempt or longer.
One of the two Superorders of ornithurine birds; it contains all birds with a palaeognathous palate—one formed by large, rigid bones (compared to a neognathous palate in which the vomer and basipterygoid process are reduced and a flexible joint exists between the pterygoid and palatine bones). Palaeognathae includes all living ratites as well as the extinct elephantbirds and moas.
See Palaeognathae.
The roof of the mouth.
Bony rod on each side of the upper jaw above the jugal arch (another set of bony rods). In birds, as the lower jaw opens it moves the quadrate, which pushes the palatine and jugal arch forward, pushing on the premaxillary bones to raise the upper jaw.
Palearctic region
Zoogeographic region that consists of most of the large landmass of Eurasia, as well as northern Africa and most of the Sahara Desert. In some systems, but not the one used by the Home Study Course, Greenland is included in the Palearctic.
One of the two groups of living birds, containing all ratites as well as the extinct elephantbirds and moas.
pamprodactyl feet
Foot arrangement in which all four toes, including the hallux, point forward. Found in some swifts.
Digestive organ and endocrine gland located in the uppermost loop of the small intestine; the pancreas secretes digestive juices into the small intestine and produces hormones (insulin, glucagon, and somatostatin) that regulate carbohydrate metabolism and blood sugar levels.
The single land mass that formed 245 million years ago from all the continents existing on Earth and persisted throughout the Triassic period.
An enormous expanse of grasslands extending across parts of Brazil, Paraguay, and Bolivia that each year is transformed into a huge marsh by torrential seasonal rains. It has one of the greatest concentrations of wildlife on the continent of South America, including numerous waders and other water birds, as well as the few remaining Hyacinth Macaws.
Distributed throughout the tropics of the world.
papilla (plural, papillae)
A small bump. 1. The bump at the end of each of a bird’s deferent ducts where it opens into the cloaca; also called the papilla of the deferent duct. When the cloaca is everted during copulation, the papillae may slightly enter the female’s ovuduct.
Tiny (microscopic) air tubes formed by the branching of secondary bronchi within the avian lung; they form a network within the lung tissue. Air carrying oxygen passes out of openings in the thick walls of the parabronchi and into a network of air spaces surrounded by a network of blood capillaries. Here, gas exchange occurs—oxygen dissolves into the blood and carbon dioxide and water vapor move from the blood into the air and back into the parabronchi, eventually to be exhaled. The avian system of airflow is much more efficient than that of mammals (in which air exchange occurs in dead-end pockets called alveoli), because in birds air flows continuously across the surface of the capillary bed.
Humid grassland area of the high Andes in South America above the tree line. It contains some shrubs and is dotted with lakes and bogs.
Describes a relationship between two species or individuals in which one benefits as a result of some cost to the other.
An ecological interaction involving two species in which one species benefits and one species is harmed.
Animals (usually insects) that feed in or on a host animal for a long period of time, consuming most of its tissues and eventually killing it. Some parasitoids (such as certain small wasps) live in the nests of cavity-nesting birds, feeding on avian ectoparasites and actually benefiting the birds.
parasympathetic system
The part of the autonomic nervous system that acts on smooth muscle to promote feeding, egg laying, and other “peaceful” activities, such as reducing the heart rate and facilitating digestion. Its nerves originate in the cranial and sacral region. The other part of the autonomic nervous system, the sympathetic system, functions under conditions of stress.
parathyroid glands
Several small endocrine glands located on or caudal to the thyroid glands; they secrete parathormone, a protein that causes calcium resorption from the bones.
parental effort or parental investment
Any expenditure of time or energy by a parent bird that benefits their current offspring at the cost of the parent’s own future survival or reproduction.
partial migration
Migratory pattern in which some individuals in a population migrate while others remain as year-round residents. When the specific individuals that migrate are determined genetically, a situation that occurs in environments where the resources always are sufficient to enable some, but not all, individuals to overwinter, the migrating birds are called obligate partial migrants; examples include the European Robin and southern European populations of the Blackcap. When the number of birds the environment can support varies from year to year, facultative partial migration evolves, in which the number and identity of the individuals migrating change from year to year in response to the availability of resources, usually food. In this case, the individuals that migrate are not determined genetically. Facultative partial migration occurs in the Blue Tit and some North American chickadees.
partial molt
A type of molt in which only some of the feathers are replaced.
One of the two large suborders of Order Passeriformes (perching birds); members are also known as oscines, songbirds, or true songbirds. Songbirds have particularly complex voice boxes, which allow them to sing more complex songs than other birds.
See passerines.
All birds in the Order Passeriformes; also called perching birds. The order contains approximately 4,600 species, nearly one-half the world’s bird species, all of which have a foot adapted for perching on branches or stems.
passive stability
when applied to flying birds, components of the bird’s aerodynamic structures that cause the bird to return to an original flight posture when it is disrupted by an outside force (such as wind) without flapping or other active movements.
A fold of tough skin that extends from the brachium to the antebrachium; the patagium connects the shoulder to the wrist and forms the leading edge of the inner wing in flight.
The kneecap; an ossification within a tendon at the lower end of the femur (thigh bone). The patella glides in a deep groove and adds stability to the knee joint.
Any organism or virus that causes disease or illness.
Highly vascularized structure of the choroid layer of the eyes of all birds and some reptiles; it projects into the vitreous body where the optic nerve exits the eyeball. The pecten is believed to nourish the retina and to control the pH of the vitreous body.
pectinate claw
A modification of the side of the middle toe claw into a comb-like, serrated edge thought to be used as a preening tool. Found in only a few birds, such as Barn Owls, nightjars, bitterns, and herons. Also called a feather comb.
pectoral girdle
The three bones (clavicle, coracoid, and scapula) on each side of the avian body that form a “free-floating” support for the wings.
pectoral veins (left and right)
Paired veins that carry blood from the pectoral region; they merge with the subclavian and jugular veins on each side to form the cranial vena cavae leading to the heart.
Large, powerful flight muscle of birds that attaches to the sternum. The pectoralis has two portions
Short, nasal vocalizations given from the ground at twilight by a male American Woodcock as part of his courtship display. They are similar to the buzzy call of a Common Nighthawk.
Of the ocean; pelagic birds spend most of their life on the open sea, feeding at the surface or just below it and coming to land only to nest. Pelagic birds include species in the order Procellariiformes, as well as tropicbirds, some penguins, the boobies and gannets, most alcids, the skuas and jaegers, the noddies, kittiwakes, Sabine’s Gull, and some terns.
pelagic feeders
See offshore feeders.
A compact ball of indigestible food, such as bones, fur, feathers, and insect exoskeletons, that is formed by the gizzard of birds that eat meat or fish—such as owls, hawks, and kingfishers—and is regurgitated through the mouth; also called a cast.
pelvic girdle
The three bones (ilium, ischium, and pubis) on each side of the body that are partly fused with one another and with the synsacrum in birds to form a strong but lightweight attachment site for the muscles of the legs, tail, and abdomen. They also provide protection for the abdominal organs. Also called the pelvis.
See pelvic girdle.
pendulous cup nest
A cup nest whose rim is supported but whose unsupported belly (nest chamber) hangs from around 4 inches (10 cm) to over 1 yard (1 m) below the supports. It is generally entered from the top. Pendulous cup nests are built by New World orioles, oropendolas, and caciques, as well as some Old World weavers, and usually are woven from plant strips or fibers. Shallow pendulous cup nests grade into pensile cup nests (see separate entry).
Describes a flat feather with distinct vanes.
pensile cup nest
A cup nest whose rim is supported but whose unsupported belly (nest chamber) hangs below. Pensile cups are built by New World blackbirds, vireos, and kinglets; by many songbirds in Australia and Asia; and by a host of orioles and flycatchers and their relatives in the Old World. Deep pensile cup nests grade into pendulous cup nests (see separate entry).
perch gleaning
See gleaning.
perching birds
See passerines.
perforate septum
A condition in which the nasal septum (tissue separating the left and right nasal cavities) has an opening or is absent. In contrast with having an imperforate septum (see separate entry), having a perforate septum appears to increase an animal’s sensitivity in detecting odors. However, it decreases the ability to locate an odor’s source, because odors entering one nostril mix with those from the other.
Thin membrane surrounding the heart; its inner lining secretes pericardial fluid, which reduces the friction of the beating heart against adjacent tissues.
The fluid that fills the structures that make up the bony outer labyrinth of the inner ear (the cochlea, semicircular canals, and vestibule), in which floats the membranous labyrinth.
A unit of geological time. Periods are divided into epochs, and successive periods make up an era.
peripheral nervous system (PNS)
The portion of the nervous system outside the brain and spinal cord; it consists of the cranial nerves as well as the spinal nerves and their associated ganglia.
Cartilaginous structure found only in songbirds; it sticks up into the syrinx at the bifurcation of the bronchi.
phagocytic cells
A cell that engulfs and absorbs potential pathogens, waste compounds, or other foreign bodies.
phalanges (singular, phalanx)
The bones of the fingers and toes.
See cloacal phallus.
Area, also known as the throat, that begins at the back of the tongue and connects the mouth and esophagus, and where the digestive and respiratory pathways cross one another. The nasal cavities and auditory tube open into the pharynx.
The pheasant family; it includes pheasants, partridges, ptarmigan, grouse, guineafowl, turkeys, and Old World quail—as well as the Red Junglefowl (the wild relative of domestic chickens) and the spectacular peafowl. Its 177 species are found throughout most of the world, but few species inhabit the Neotropics.
The tendency of individuals of certain species to eventually take up residence (and breed) near the area where they were born.
The amount of a 24-hour day in which birds are exposed to light.
A behavioral or physiological response to day length.
phylogenetic species concept
A species concept that focuses on the history of a related set of organisms, in which a species is usually defined as the smallest group descended rom a common ancestor that share some kind of distinguishing feature that separates them from other such groups.
phylogeny (or phylogenetic tree)
A diagram that depicts the evolutionary relationships connecting a set of organisms.
The scientific study of the evolutionary relationships of populations and species across space, usually as inferred from genetic information.
phylum (plural, phyla)
Level of classification of organisms above “class” and below “kingdom”; similar classes are placed within the same phylum. All birds are in the phylum Chordata.
physiographic regions
Regions of the world that are biologically distinct based on climate conditions, topography, soil types, and plant communities; also called ecoregions.
Microscopic plants and algae that float freely in aquatic environments; phytoplankton make up the “plant” portion of plankton.
The innermost of the three vascularized membranes called meninges that surround the brain and spinal cord. The meninges provide sustenance and waste removal for the cells of the brain and spinal cord, which are not served by the circulatory system.
pigeon’s milk
See crop milk.
Colored substances that give color to the structures, such as skin and feathers, in which they occur. In theory, pigments can be extracted from these structures.
pin feathers
Developing feathers that are still surrounded by a feather sheath.
pineal gland
Endocrine gland located in the dorsal midbrain; it secretes the hormone melatonin, which plays a role in regulating daily activity cycles (circadian rhythms).
pinyon-juniper woodland
Ecosystem of southwestern North America consisting of park-like stands of small pinyon pines and junipers growing on hills and mountain slopes below the montane forests. Five species of breeding birds are particularly characteristic to this dry ecosystem
The condition of an egg that is approaching hatching, in which the embryo within has punctured the first small hole through the shell.
A foraging technique in which a bird steals food from another bird; used by Parasitic Jaegers, frigatebirds, and some gulls, among others.
See frequency.
An Old World family (Pittidae, 31 species) of secretive, stocky suboscines with long legs and a short tail; they live on the tropical forest floor, and many are brightly colored below and cryptic above. Pittas use their heavy bills to catch a variety of insects and other small animals, especially snails.
pituitary gland
An endocrine gland attached to the ventral surface of the hypothalamus; it consists of an anterior and a posterior lobe. The pituitary synthesizes, stores, and releases a wide variety of hormones.
Microscopic plants (and algae) termed phytoplankton and microscopic animals (and protozoa) termed zooplankton that float freely in aquatic environments.
The fluid portion of the blood; it contains sugars, inorganic salts, and certain proteins (plasma proteins) found only in the plasma, and carries numerous types of dissolved substances. Avian plasma has a higher sugar, fat, and uric acid content than the plasma of most mammals.
platform nest
Nest consisting of a very shallow depression in the top of a mound of nest material. Varies in complexity from the flimsy platform of twigs built by Mourning Doves to the enormous stick piles amassed by storks or large birds of prey such as Osprey and eagles. Some platform nests, such as those of some terns and rails, float.
playback experiments
Experiments in which researchers play songs (through a speaker) to territorial birds, and then note their responses.
Complex web- or net-like structures along the spinal cord at the level of the front limbs or wings (brachial plexus) and hind limbs (lumbosacral plexus); they are formed by a merging of the large spinal nerves within each region.
Refers either to a bird’s entire feather coat, or to the set of feathers produced by a particular partial or complete molt.
Describes a feather on which the fluffy barbs are not structured into flat vanes.
plunge diving
A foraging technique in which an airborne bird dives under water to pursue aquatic prey; performed by birds such as Brown Pelicans, auks, gannets, Osprey, and kingfishers.
Describes bones that are filled with air spaces and may contain air sacs, which are nearly all the bones of most birds.
Describes bones that are filled with air spaces and may contain air sacs; nearly all the bones of most birds are pneumatic.
pneumatic foramen
Opening in the avian humerus (upper wing bone) leading to the air space within the bone; extensions of the clavicular air sac lead into the pneumatic foramen and thus connect with the air space.
Filled with small air cells.
See peripheral nervous system.
The tough skin covering the tarsus.
point counts
A popular census method for birds in which an observer notes the birds seen or heard from a stationary point over a standard period of time.
polarized light
Light in which the waves are all oriented in the same plane, or in a reduced number of planes, compared to unpolarized light.
polarizing lens or filter
Something that selectively transmits only those light waves with certain orientations, creating a polarized beam of light.
Mating system in which one female mates with several males within the same breeding season.
General term for a mating system in which individuals of one sex (either the males or females) mate with more than one partner during the same breeding season. Both polygyny and polyandry are forms of polygamy.
Mating system in which one male mates with several females within the same breeding season. Polyandrous species include Spotted Sandpipers, most jacanas, and certain phalaropes.
A situation in which one species contains two or more distinct morphological types (morphs) of individuals, which are determined genetically. Morphs may differ in physical characteristics such as color, body size, or bill length or shape, but not in characteristics that are related to sex, age, locality, or season.
In ornithology, a group of interbreeding birds of the same species that live in the same place at the same time.
population connectivity
The degree to which two breeding populations are connected by dispersal and gene flow, often through contact of individuals in common wintering areas.
population density
The number of individuals per unit area in a given population.
population dynamics
Changes in population size, density, dispersion, and structure over time.
population ecology
The study of how animal populations are related to, and respond to, their environments. It involves monitoring and studying reproductive rates, survival rates, movements of individuals and populations, and changes in population densities over time and from one area to another.
population structure
The relative proportions of males and females, individuals of different ages, and individuals with other definable differences in a population.
population viability analysis
A modeling technique used by conservation biologists to identify the biological and human factors most strongly affecting the long-term survival of a population or species, and to estimate how population persistence varies with population size.
Pigments producing red, brown, green, or pink colors in a number of avian orders; they are especially well known for producing the bright reds and greens of the turacos. Porphyrins are complex, nitrogen-containing molecules related to hemoglobin, that birds and other organisms synthesize by modifying amino acids.
Porro prism binoculars
Binoculars in which the eyepieces are closer together than the objective lenses.
portal system
A blood circulatory pattern involving portal veins.
portal veins
Veins (such as the hepatic portal vein and the renal portal vein) that carry blood between two capillary beds, rather than connecting a capillary bed and the heart, as most veins do. Portal veins allow a second organ to process the blood before it is returned to the heart and thus the general circulation.
Toward the back of an organism, using the earth as a frame of reference. With birds, technically only used within the eye and inner ear. But in practice, often used interchangeably with cranial.
posterior air sacs
General term referring to the air sacs nearest the bird’s caudal end—the posterior thoracic and abdominal air sacs.
posterior chamber
Space between the iris and lens of the eye; it is filled with aqueous fluid, which nourishes the eye and removes wastes.
posterior lobe of the pituitary gland
Part of the pituitary gland that does not manufacture hormones itself, but instead stores and releases neurohormones such as mesotocin and arginine vasotocin.
post-hatch brood amalgamation
A broad term describing a variety of methods by which young from several broods may combine into larger groups.
A tough band of tendinous tissue running along the edge of the antebrachium and manus that trails in flight. The postpatagium holds the primary and secondary feathers firmly in place and supports each quill.
powder downs
Down feathers that are never molted but grow continuously, disintegrating at their tips to produce a fine powder resembling talcum powder, which may help to waterproof the feathers.
powered flight
Type of flight in which the flying object produces thrust. In a bird, refers to flight powered by flapping, which produces thrust to propel the bird forward.
Describes young birds that hatch in a relatively developed state—downy and with their eyes open. Many are soon able to walk or swim and even eat on their own.
precopulatory displays
Displays given by either sex immediately before copulation that appear to invite or solicit copulation. Female birds of many species perform a stereotyped precopulatory display in which they raise the tail and hold the wings down or out, quivering them.
preen gland
See oil gland.
Feather maintenance behavior in which a bird grasps a feather near its base, then nibbles along the shaft toward the tip with a quivering motion; this cleans and smooths the feather. Many birds gather on their bill oily secretions from the oil gland, and then spread them on their feathers as they preen.
Main bone forming each side of the upper beak of a bird. The two halves together are called the premaxillary bones.
prescribed burning
Purposefully starting fires under controlled conditions, at intervals that mimic the natural fire cycle of the landscape, to maintain certain types of habitats and the species that depend on them.
prey switching
A behavior in which predators preferentially consume the most abundant type of prey, switching to another prey type when it becomes the most abundant.
The flight feathers of the outer wing that are attached to the manus.
primary bronchus
See mesobronchus.
primary consumers
Level in a food chain or web that consists of organisms—usually animals—that directly eat plants or other producers; most primary consumers are herbivores.
primary feathers
The flight feathers of the outer wing; they are attached to the manus.
primary productivity
Gross primary productivity is the rate at which green plants and the few other producers (such as green algae) in an ecosystem produce biomass, including the biomass that they use themselves during respiration. Net primary productivity is the gross primary productivity minus the biomass that the producers use themselves; in other words, it is the rate at which the producers create biomass that is available as food for other organisms.
primary succession
Ecological succession that begins on a substrate, such as rock, sand, or lava, that has never before supported a community. Also refers to the process by which a lake gradually fills in to form a bog community. See ecological succession.
primitive streak
In avian biology, very early embryonic stage consisting of a long trough (the “streak”) with raised sides that establishes the cranial-caudal, left-right, and dorsal-ventral body axes. The primitive streak is formed in the flattened mass of cells that lies on the upper surface of the yolk.
principle of allocation
The assumption of a trade-off in energy and other resources, such that those devoted to one aspect of life history (such as survival) thereby become unavailable for others (such as reproduction).
Level in a food chain or web that consists of green plants and a few other organisms (such as green algae) that capture energy from the sun and then introduce the energy into the food chain by incorporating it into organic compounds, which the producers synthesize from water and inorganic substances through the process of photosynthesis.
In biology, the total number of grams of living material produced per square meter per year (g/m2/yr) in a particular area or habitat. Productivity includes the increase in size of all living things, as well as new individuals added through birth or germination. Productivity may be measured as gross productivity or net productivity (see similar definitions under primary productivity).
projectile vomiting
Vomiting that is sufficiently forceful to propel the regurgitated substance a short distance away. Birds such as vultures, herons, gulls, and tubenoses (Fulmars, albatrosses, petrels, storm-petrels, and diving petrels) use this technique to ward off predators; Fulmars can spew an oily, foul-smelling mixture of flesh and fluid several feet.
A “body-parts-position sense”
Nerve stimuli that give information on the position and movement of the body or parts of the body.
Complex molecules composed of strings of amino acids; proteins are the main building blocks of all living organisms and also act as enzymes, assisting chemical reactions.
The living material composing all organisms.
The upper part of the bird’s two-part stomach; it is elongate and glandular, and secretes mucus, hydrochloric acid, and an inactive precursor to pepsin, an enzyme that digests protein.
Toward the center of the body (the elbow is proximal to the fingers) or toward the origin of the structure (the base of a feather—where it is attached—is proximal to its tip).
In biology, pertaining to immediate, internal causes; mechanistic. Proximate questions about a behavior, for example, look into the internal causes of the behavior—how it is carried out. For comparison, see ultimate.
proximate factors
In biology, the actual cues that trigger the body to carry out a particular process or behavior. For example, day length is a proximate factor that affects the timing of breeding in birds. For comparison, see ultimate factors.
proximate questions
In biology, questions that pertain to immediate, internal, and/or mechanistic processes, often addressing “how” the process is produced by physiological, behavioral, or other means.
A family of 13 sparrow-like birds with slender, pointed bills; it includes the accentors and Dunnock. Prunellidae is the only bird family endemic to the Palearctic region.
A foraging technique in which a bird bites off and eats plant buds; used by Ruffed Grouse and other birds.
A family (Pomatostomidae, 5 species) of noisy, busy, social ground-feeding songbirds with long, curved beaks and long, towhee-like tails; they are endemic to the Australasian region.
pseudosuchian thecodont
Term used primarily by Robert Broom in his “pseudosuchian thecodont hypothesis” to refer to the thecodonts (see separate entry).
pseudosuchian thecodont hypothesis (of bird evolution)
Theory that birds evolved approximately 230 million years ago from small, arboreal thecodonts; first proposed by Robert Broom in 1913.
Flying reptiles from the Triassic that radiated and diversified in the Jurassic and Cretaceous. They had features convergent with those of birds, such as hollow bones and a slight keel on the sternum, but their large, membranous wings were structurally unique
pterylae (singular, pteryla)
Areas of a bird’s skin where feathers are attached; also called feather tracts.
The arrangement of feather tracts and bare patches on a bird.
One of the three paired bones that fuse to form the pelvic girdle; the long, thin pubis runs caudally along the ventral side of the ischium, and is only partly fused with it.
puddle ducks
See dabbling ducks.
pulmonary arteries
Two large arteries (right and left) carrying oxygen-poor blood from the pulmonary trunk (coming from the right ventricle of the heart) to the lungs; the pulmonary arteries and their branches are the only arteries that carry deoxygenated blood.
pulmonary arterioles
Tiny arteries within the lung that supply deoxygenated blood to the capillaries surrounding the network of air spaces adjacent to the parabronchi, where air exchange occurs in birds.
pulmonary circulation
Portion of the circulatory system carrying blood from the heart to the lungs and back to the heart.
pulmonary trunk
A large artery that carries oxygen-poor blood from the right ventricle of the heart and then branches to the right and left lungs as the right and left pulmonary arteries.
pulmonary veins
Two large veins (left and right) carrying oxygen-rich blood from the lungs to the left atrium of the heart; the only veins that carry oxygenated blood.
pulmonary venules
Tiny veins within the lung that carry oxygenated blood away from the capillaries surrounding the network of air spaces adjacent to the parabronchi, where air exchange occurs.
The opening in the center of the iris; it controls the amount of light entering the eye.
The yellowish fluid seen in infections; it is a mixture of bacteria, dead white blood cells, and fluid.
See population viability analysis.
The tail bone of birds; it is formed by the fusion of the final few caudal vertebrae. The pygostyle is shaped like a plowshare and provides an attachment site for the flight feathers of the tail.
A muscular sphincter (circular band of muscle) at the lower end of the stomach; it regulates the passage of food from the stomach into the small intestine.
Pear-shaped. Describes an egg that is rounded at one end and relatively pointed at the other; also called conical.


quadrate bone
Bone on each side of the skull, at the caudal and lower end of the upper jaw; in birds, it links with the articular bone on each side of the lower jaw. This linkage forms the joint between the upper and lower jaws, allowing a bird to open its mouth widely.


r and K selection
Ecological terms for the ends of a continuum between extreme investment in reproduction at the cost of long-term survival (r selection), and extreme investment in survival at the cost of high fecundity (K selection).
A population of a species, usually in a particular geographic area, that is morphologically distinct from other populations of the same species, but whose members remain capable of interbreeding with members of those other populations. Also called a subspecies.
The portion of the feather shaft above the calamus; it is solid and has a vane on each side.
The larger of the two wrist bones of birds; it consists of fused carpals.
The smaller of the two bones supporting the antebrachium (forearm).
The central shaft of a feather barb.
The geographic area within which a species or population generally remains at a particular time of the year; a species may have different breeding and nonbreeding ranges. Also called the geographic range.
range disjunction
A pattern of geographic distribution in which two or more occupied areas are separated by substantial intervening areas where the species is not found.
Members of the orders Falconiformes and Strigiformes, which contain all the diurnal and nocturnal birds of prey.
All birds lacking a keel on the sternum. Includes flightless birds—the Ostrich, rheas, Emu, cassowaries, and kiwis—as well as the tinamous, which are fully capable of flight.
A harsh trill.
realized niche
The subset of its fundamental niche that a species actually occupies.
The establishment of a population of a particular species in an area where that species formerly occurred, generally through natural dispersal.
rectrices (singular, rectrix)
The long, stiff flight feathers of the tail.
recurrent bronchi
Secondary bronchi (branches off the main air tube, the mesobronchus) within the avian lung that connect to the air sacs outside the lung.
Curved upward. Describes beaks such as that of the American Avocet.
red blood cells
Flattened, elliptical, red-colored cells found in the blood; they house the iron-containing pigment hemoglobin, which carries large amounts of oxygen and gives the blood its red color. The red blood cells of birds have a nucleus, unlike those of mammals. Also called erythrocytes.
red fibers
A type of muscle fiber that appears red because it contains a large amount of myoglobin (which carries oxygen) and is permeated by massive capillary beds (containing many red blood cells, which also carry oxygen). Muscles with many red fibers produce energy aerobically (using oxygen), without building up the quantities of lactic acid that cause fatigue, and thus can sustain actions for long periods of time. Therefore, long-distance fliers often have a preponderance of red fibers. Muscles with many red fibers often are called “dark meat” (as in the drumstick of a turkey).
redirected activities
Actions that are appropriate for a given situation, but are directed at an inappropriate subject; often they appear at times of stress or conflicting motivations. Some have been exaggerated and incorporated into displays.
A relatively simple and stereotyped response to a stimulus; it may be either automatic or learned.
Areas remaining stable when surrounding areas are undergoing change (such as that caused by glaciation); often used to describe isolated patches of rain forest separated by grasslands during glacial periods.
regional heterothermy
The ability to maintain core body temperature while allowing the temperature of the extremities to deviate from the core temperature.
regional population
A population that inhabits a fairly large area; for example, all the Canada Geese in the central migratory flyway.
A situation in which natural selection fosters the divergence of two hybridizing populations, either by selecting against hybrid offspring directly or by favoring individuals that choose members of their own population as mates.
The establishment of individuals of a species (through human effort) in an area where that species used to live—using individuals from a different area (translocation) or from a captive breeding program.
relative abundance (of species)
The number of individuals of each species living in a particular community or area compared to the numbers of individuals of other species in that area. Relative abundance measures how evenly individuals are distributed among species.
Specific objects, physical features, or behaviors that activate (or “release”) a specific response in an individual.
relict population
A small, isolated population that is a remnant of a much larger population that once existed over a wider geographic area.
remiges (singular, remex)
The longest wing feathers, also called the flight feathers of the wings; they include the primary and secondary feathers.
renal efferent veins
Veins that collect blood from the capillary beds of the kidneys and carry it to the caudal vena cava, which returns it to the heart.
renal portal system
System of blood flow, found in birds and reptiles but not in mammals, that collects venous blood from the lower portion of the digestive tract via the caudal mesenteric vein, and conveys it to the venous ring within the kidney, where some of it passes through a capillary bed before being conveyed back to the heart. The reason for this pattern of blood flow is unknown.
renal portal veins (left and right)
Veins carrying blood from the caudal mesenteric vein to the capillary beds of the two kidneys; part of the renal portal system of birds.
See song repertoire
reproductive isolation
A mating incompatibility the occurs when some kind of behavioral, physiological, or genetic barrier prevents successful interbreeding among individuals from two different populations even where they occur together.
reproductive retaliation
A behavior in which one member of a pair provides less parental care in situations where the offspring in the nest may not be their own and/or when their mate also provides less care.
reproductive skew
Refers to the degree to which reproduction is limited to only a few adults in the population (high skew) or is more equal among all adults in the population (low skew).
reproductive success
The number of viable offspring produced by an individual over their lifetime.
reproductive value
The expected reproduction of an individual from their current age onward, given that they have survived to their current age.
Individuals that live year round in a particular area.
resource-defense polygyny
Mating system in which one male may have several mates, and the females choose their male partners by evaluating the quality of their breeding territories; males with better territories attract more mates. This system may evolve when safe nesting sites or rich food supplies are more important to the survival of the young than is male parental care. Found in Red-winged Blackbirds and a few other species.
reticulate podotheca
A podotheca that is divided into a network of small, irregular, nonoverlapping plates.
The innermost layer of the eyeball, upon which images are focused; it is pigmented and contains the light-sensitive rod cells and cone cells.
retort nest
A globular nest with an entrance tunnel; built out of mud by many mud-nesting swallows such as Cliff Swallows, and constructed from grass by many African weavers and New World swifts in the genus Panyptila.
reverse Porro prism binoculars
Binoculars in which the objective lenses are closer together than the eyepieces; characteristic of some compact binoculars.
reverse sexual dimorphism
A situation in which the sex that is usually smaller or less ornamented is the larger or more conspicuous sex in a particular species. In birds, sexual selection is usually stronger on males, so reverse sexual dimorphism refers to species where females are the more flashy sex.
reverse sexual size dimorphism
A situation in which females are significantly larger than males of the same species; found in many predatory birds such as raptors, jaegers, skuas, frigatebirds, and boobies. Because the sexes are different sizes, they can take different sizes and thus different types of prey. This may reduce competition between mates, but why females of these species are larger than males (rather than the more typical situation, in which females are smaller) remains a matter of much speculation.
Outer covering (sheath) of the beak.
Outer covering (sheath) of the beak; it grows throughout a bird’s life. In most birds it is hard and horny, but in waterfowl, pigeons, and some shorebirds it is soft and leathery.
Two species (family Rheidae) of large, flightless ratites of temperate open country in South America.
rhomboid sinus
Opening on the dorsal midline of the lumbosacral enlargement of the spinal cord; it contains a gelatinous mass of neuroglial cells rich in glycogen, known as the glycogen body, whose function is unknown. The rhomboid sinus and glycogen body are unique to birds.
The ability of some birds to independently move or open the upper tip of a long bill, most often seen in birds that feed by probing in soil or mud.
rictal bristles
Stiff, hairlike feathers projecting from the base of the beak in birds that catch insects. They may protect the face and eyes of some birds that capture large, scaly insects; they also may help birds detect movements of prey held in the beak.
right heart
The right atrium and right ventricle of the heart; these chambers contain deoxygenated blood, which does not mix with the oxygenated blood of the left heart (left atrium and ventricle).
The evolutionary process by which certain everyday motions become exaggerated, repeated, and stereotyped into displays presenting a clear message.
Dull-colored African songbirds (family Picathartidae, 2 species) with colorful, bare heads; they hop along the rain forest floor in rocky areas and nest colonially in caves.
One of the two kinds of light-sensitive cells lining the retina of the eye; they are particularly good at detecting low light levels (but not in differentiating colors, which is the function of the cones). When a rod cell is stimulated by light energy, it sends a nerve impulse to the brain via the optic tract.
Old World nonpasserines (family Coraciidae, 12 species) colored in shades of blue, pink, olive, or chestnut; they are named for their rolling or rocking dives during courtship flights. They have long, slender beaks and forage by perching and then flying to the ground to catch large arthropods.
roof prism binoculars
Binoculars with straight barrels, in which the eyepieces and objective lenses are directly in line.
Toward the beak; used for positions on the head and neck.
round window
Former name of the cochlear window.
r-selected species
Species in which individuals have a life history strategy that emphasizes high rates of reproduction instead of (and probably at the expense of) long-term survival. These species are generally small and tend to develop rapidly, breed at an early age, have many young per cycle (large litters or clutches), breed frequently, care for their young only briefly, and have short life spans.
The lower back of the bird, above the tail coverts; conspicuously colored in some birds.
runaway sexual selection
A hypothesis for how extremely exaggerated ornaments or behaviors might evolve, via a feedback loop that involves females choosing males with the extremes of that trait, the resulting generations of male offspring having more and more elaborate variants of the trait, and the resulting female offspring having ever stronger preference for males with the trait.


One of the two membranous chambers inside the bony vestibule of the inner ear (the other is the utriculus); these chambers contain hair cells and dense crystals called statoconia, both of which are embedded in a gelatinous material that is surrounded by endolymph. The statoconia and hair cells perceive the position of the head with respect to gravity (see entry for statoconia).
sacral vertebrae
The vertebrae of the pelvic region; in birds, they are all fused with the lumbar vertebrae and with some of the thoracic and caudal vertebrae to form the synsacrum.
sagebrush ecosystem
Fairly dry ecosystem found above the lowest elevations of the Great Basin of North America; it is a shrubland dominated by sagebrush, shadscale, and other woody growth 2 to 5 feet (0.6 to 1.5 m) high. Of the few breeding birds, four species are particularly characteristic
sagittal plane
A vertical plane through the long axis of an organism, extending from head to tail. It divides the body into left and right portions.
salivary glands
Glands that secrete saliva, which primarily acts to moisten food in the mouth. Some birds that obtain their food from the water, such as Anhingas, have no salivary glands. The salivary glands of others, such as certain swifts and swiftlets, secrete an adhesive substance used in nest building.
sally gleaning
See gleaning.
salt glands
Paired glands that extract and excrete excess salt from the blood; most conspicuous in marine birds, especially the “tube-nosed” seabirds.
Sixteen bird species that form the family Pteroclidae and that inhabit the deserts and dry savannas of Africa and Eurasia; they are similar in size and shape to pigeons, but taxonomists are not sure of their relationships to other families, and thus their order is not determined. Sandgrouse feed primarily on dry seeds and often fly long distances to water holes to obtain water. They are well known for their specialized belly feathers, which are structurally adapted to absorb and retain water. Parents (usually the male) soak the belly feathers at a water hole, and then carry the water back to their nestlings.
sap well
A rectangular hole up to a few inches long, drilled through the bark of trees and shrubs by a sapsucker. The bird may return to the well many times to feed, both on the sap that flows from the wound and on insects that are attracted to the flowing sap. Other birds and mammals also may feast at the sapsucker’s well.
saurischian dinosaurs
One of the two major groups of dinosaurs, the saurischian dinosaurs are also known as “reptile-hipped” dinosaurs because of the structure of their hip joint. They are composed of two groups, the herbivorous sauropods and the carnivorous theropods.
One of the two major subclasses of birds; it contains primitive birds such as Archaeopteryx and the enantiornithines (opposite birds), but no living bird groups.
Describes birds in the Subclass Sauriurae.
The shoulder blade; one of the three sets of paired bones that make up the pectoral girdle.
A group of feathers in the shoulder region.
Organisms that eat dead organisms and wastes.
scientific method
A procedure that scientists use to investigate how the world works. The specific steps typically followed by scientists to investigate aspects of the world vary among the different scientific disciplines, but in the biological sciences the scientific method usually involves the following
Tough, whitish, outer layer of connective tissue surrounding the eyeball.
scleral ossicles
Bony rings supporting the eyes of all birds, lizards, turtles, and fishes; they are not present in mammals.
A shallow depression created by certain types of birds for use as a nest. It usually contains little or no nest material; in some species it may be lined with a few flat pebbles. Scrapes are built by many plovers, terns, skimmers, and penguins.
A South American family (Anhimidae, 3 species) of heavy-bodied, goose-like birds with far-reaching calls.
scrub desert
Hot, dry ecosystem of the lowlands and valley floors of southwestern North America; it is composed of bare ground with widely spaced plants such as creosote bush, ocotillo, and bur yucca. The scrub desert ecosystem hosts a moderate number of breeding birds, which tend to synchronize their nesting with periods of rainfall.
scutellate podotheca
A podotheca that is broken up into overlapping scales.
All birds directly associated with the open seas and consistently dependent on the seas for food; also called marine birds.
search image
A general idea or internal representation of an object being sought that allows an animal to find it easily. If an animal has a search image of a particular type of prey, it is able to find the prey more efficiently than it can find other prey for which it has no search image.
The flight feathers of the inner wing that are attached to the ulna.
secondary bronchi
Branches off the mesobronchus (main air tube) of each avian lung; some connect to the air sacs and are called recurrent bronchi, and others divide to form the tiny parabronchi, the major respiratory units of the avian lung.
secondary cavity nesters
See cavity adopters.
secondary consumers
Level in a food chain or web that consists of organisms—usually animals—that directly eat primary consumers.
secondary feathers
The flight feathers of the inner wing; they are attached to the ulna of the antebrachium.
secondary sex characters
Anatomical features that distinguish the sexes, but are not directly involved in the production of eggs or sperm.
secondary succession
Ecological succession (see separate entry) that begins with bare soil or an existing community.
A long-legged bird of prey that stalks the African savannas on foot for snakes, other small vertebrates, and large insects. It is the only member of its family, Sagittariidae.
Section 404, Clean Water Act
Part of the Clean Water Act (see separate entry) passed in the United States in 1972; Section 404 strives to protect wetlands by regulating the filling of, or discharge of dredged material into, all fresh waters of the United States, including wetlands. It allows wetland filling by permit if applicants show that less damaging alternatives are not feasible, and that the water will not be degraded significantly. Applicants must avoid wetland impacts when possible, minimize wetland damage, and provide compensation (wetland mitigation—see separate entry) for all wetland impacts deemed unavoidable.
A family (Thinocoridae, 4 species) of ptarmigan-like birds that nest both in the southern lowlands and the high mountains of South America.
semicircular canals
Three bony, ring-like canals of the inner ear, arranged at right angles to one another (one in each plane of space); they are part of the bony labyrinth, and thus are filled with perilymph. They enclose the semicircular ducts, which contain the hair cells that sense changes in the organism’s speed or direction, thus guiding balance.
semicircular ducts
Three ducts floating in the perilymph of the semicircular canals of the inner ear, one in each plane of space; they contain the hair cells that guide balance by sensing changes in the organism’s speed or direction. They are part of the membranous labyrinth of the inner ear and thus are filled with endolymph.
semilunar membrane
Membrane that extends from the pessulus into the cavity of the syrinx; it is found only in songbirds.
semilunar valves
Valves, each composed of three cusps (flaps), that are located at the beginning of the pulmonary trunk and the aorta; the semilunar valves prevent the backflow of blood into the ventricles as the ventricles relax after each heartbeat.
Feathers that occur in a continuum of forms between down and contour feathers. Located at the edges of the contour feather tracts, semiplumes provide insulation and help to maintain the bird’s streamlined shape.
A decline with age in physical, physiological, and/or reproductive performance, or an increase with age in the probability of death.
sensitive period
A relatively brief span of time early in life during which songbirds are best able to memorize the details of songs they hear.
sensory neuron
Nerve cell that conveys impulses to the spinal cord and brain; these impulses are interpreted as sensations—either conscious or subconscious.
sentinel system
A system in which group members take turns watching for danger. Birds using sentinel systems include Florida Scrub-Jays and some American Crows.
Tissue that extends ventrally from the upper wall of the nasal cavities and separates the left and right nasal cavities from one another; may be perforate or imperforate (see separate entries).
sequential polyandry
A mating system in which one female mates with multiple males within a breeding season, but with only one male during each successive nesting attempt.
A family (Cariamidae, 2 species) of fast, long-legged, grassland birds of South America; seriemas chase down their reptile prey on foot.
sex hormones
Hormones produced by the anterior lobe of the pituitary gland, the adrenal glands, and (primarily) the gonads; they regulate the anatomical structures, physiological processes, and behaviors that are essential for reproduction.
sex ratio
The ratio of males to females (or females to males)—usually noted for a given population.
sexual dimorphism
A situation in which males and females of the same species differ from each other in size or form.
sexual selection
A special form of natural selection that involves the differential reproductive success of individuals that arises specifically from competition over mating opportunities; this can involve direct competition with members of the same sex or competition to attract members of the opposite sex.
sexy son hypothesis
The idea that breeding females can achieve higher fitness by mating with particularly attractive males because their sons are thereby more likely to inherit those attractive traits and be reproductively successful themselves.
The stiff, central rod down the center of a feather, from which the vanes extend.
The temporary tube-like covering that surrounds a growing feather.
shell gland
Region of the avian oviduct after the isthmus and before the vagina; it is lined with papillae that secrete fluid albumen and the calcium-rich shell around the fertilized ovum. Shell pigments, if any, also are added here; the patterns of pigmentation reflecting the speed of the egg’s passage and rotation through the region.
Uncoordinated muscle fiber contraction; a way to produce heat without directed movement by muscle contraction.
shock molt
A rare situation in which a bird sheds many feathers all at once; it usually is the result of stress, such as when a bird is grabbed by a predator (tail feathers may be shed if grabbed by the tail), handled by humans, or exposed to violent natural events such as earthquakes and tornados. Also called fright molt.
A large, stork-like water bird endemic to Africa; it has a huge, shoe-shaped, hooked bill for seizing large fish. It is the only member of its family, Balaenicipitidae.
shore feeders
Birds (or other animals) that hunt on shore in the intertidal zone (between the low and high tide marks), relatively close to their nests; shore feeders include marine waders, such as the American Oystercatcher, and many other shorebirds.
Oystercatchers, plovers, snipes, sandpipers, curlews, phalaropes, and sheathbills. Ornithologists in Britain and the British Commonwealth, except Canada, speak of shorebirds as “waders.”
The killing of one’s sibling; in birds whose offspring practice siblicide, usually the stronger, older nestlings kill the younger, weaker nestlings, either by direct attack or by pushing them out of the nest.
A foraging technique in which a bird sweeps the partly opened bill from side to side in water or mud, straining out small animals and plant material with the specialized edges of the bill as the water (or mud) drains out. Sifting is used by Roseate Spoonbills, Northern Shovelers, and others.
single-population models (in PVA)
See population viability analysis.
sink population
A population in which reproduction is not sufficient to maintain population size, and which therefore must receive immigrants dispersing from other populations to remain extant.
sister taxa
Two groups (often but not always two species) that are each other’s closest evolutionary relatives.
site fidelity
Loyalty shown by birds or other organisms to places they previously occupied; the places may be breeding locations, nonbreeding locations, or stopover points between the two. Also called site tenacity.
site tenacity
See site fidelity.
skeletal muscles
Muscles that move the bones; they constitute the “meat” of an animal and are under conscious control. Also called voluntary muscles.
A technique by which field workers use the degree of skull ossification and pneumatization to determine the age of live passerines captured in the fall.
slope soaring
A type of static soaring in which a bird derives lift from air deflected upward when wind strikes a hill, ridge, or cliff; or from rising eddies created when wind spills over a cliff. Slope soaring is particularly common along seacoasts among gulls, terns, fulmars, and gannets; it also is common along mountain ridges, where ravens, crows, and migrating hawks may soar.
Gaps between the feathers of the wing tips, created when a bird having narrow-tipped primaries spreads them during flight. Slots are most common in large, soaring birds, such as vultures, condors, eagles, and certain hawks, because they allow these birds, which have broad, rounded wing tips, to increase their lift-to-drag ratio. Slots reduce tip vortex by turning each primary feather into an individual, narrow, pointed “wing tip”; they also increase lift because each separated primary feather acts as an individual airfoil.
slotted high-lift wings
Broad wings with a deep camber and very prominent slotting; the broadness reduces wing loading and allows low flight speeds, and the slotting increases lift, allowing the birds to carry heavy prey. Birds such as vultures, eagles, condors, storks, and some owls and hawks have this type of wing; they use it for static soaring.
small intestine
The longest part of the digestive tract, running from the stomach to the large intestine. The final processes of digestion take place inside the small intestine, as proteins are broken down into amino acids, carbohydrates are converted to simple sugars, and fats are reduced to glycerol and fatty acids. The small intestine is longer in birds that feed on foliage or grain than in those that feed on fruit or meat, reflecting the difficulty of digesting the cellulose in plant material.
smooth muscle
Type of muscle found in the walls of the hollow organs (such as the stomach or intestines) and in the walls of blood vessels larger than capillaries, especially arteries and arterioles. The contraction of smooth muscle is not controlled consciously.
The limp, red, fingerlike, fleshy structure that projects over the bill from the forehead of a Wild Turkey.
In a bird or other animal, flying without flapping the wings or limbs, while gaining altitude or remaining horizontal (as compared to gliding, in which the animal loses altitude).
social foraging
See cooperative foraging.
social mate
The individual with which a bird has a pair-bond.
social monogamy
In birds, a mating system in which one female forms a lasting pair-bond with one male, though she may also copulate with other males.
Solnhofen limestone
Fine-grained limestone deposits near the village of Solnhofen in Bavaria that, due to meticulous mining, have produced an array of amazingly well-preserved fossils of plants, invertebrates, fishes, and reptiles, as well as several specimens of Archaeopteryx.
See cell body.
In avian biology, paired segments that appear along the middle region of the cranial-caudal axis of the very early embryo; eventually they will become vertebrae and muscle.
A visual representation of sounds, such as bird song, plotted as a graph of frequency or pitch (vertical axis) versus time (horizontal axis). The darkness of the images gives a rough indication of the relative loudness of the different sounds.
song control centers
Groups of nerve cells within the brains of songbirds that are involved with both the hearing and production of songs; they are interconnected with one another in a series of complex pathways. Individuals that sing frequently or that have large song repertoires have large song control centers. The centers shrink in the nonbreeding season—a time when most birds sing less often.
song repertoire
All the songs (different song types) sung by an individual bird; for comparison, see vocal repertoire.
Members of Suborder Passeri, which is one of the two large suborders of Order Passeriformes (perching birds); songbirds also are known as oscines or true songbirds. Songbirds have particularly complex voice boxes, which allow them to sing more complex songs than other birds.
Loud vocalizations, often delivered from an exposed perch, that are presumed to attract mates or repel territorial intruders. Strictly speaking, songs are given only by songbirds—passerines in the suborder Passeri (Oscines). In practice, however, similar vocalizations made by nonsongbirds (and even other animals) often are called songs.
source population
A population in which more offspring are produced than can breed there, leading some to disperse away to other locations.
Southern Marine Region
The major faunal region of the seas that includes the frigid waters around Antarctica north to about 35 degrees south latitude.
southwestern oak woodland
North American ecosystem that is scattered on hills and mountain slopes and that contains open woodlands consisting primarily of oaks and occasional large ponderosa pines; southwestern oak woodland is found in Utah, Nevada, California, New Mexico, Arizona, and parts of Colorado. Rainfall is low to moderate.
spatial learning
Learning the location of objects.
spatial synchrony
Occurs when the different sub-populations within a larger metapopulation tend to grow or shrink in size at the same times.
speaker replacement experiments
Experiments in which researchers remove animals (usually male birds) from their territories, and then replace them with tape recorders and loudspeakers broadcasting various sounds (the male’s song, the song of a different species, background noise with no song, and so on), depending on the specific experiment. Then the researchers watch the reaction of other birds in the area.
An organism that uses only a narrow range of some type of resource; for example, birds with specialized diets eat only one or a few kinds of foods.
The evolutionary process by which one ancestor lineage splits into two or more descendant species.
The basic unit of biological classification for birds. Criteria for defining bird species vary, but they generally group into a single species the individuals and populations with very similar traits, a history of recently shared ancestors, and the continued ability to fully interbreed.
species account
A set of written field notes on a particular species.
species accumulation curve
A graph that shows the number of species detected on the y-axis and the number of survey samples or individuals observed on the x-axis.
species assemblage
Similar to “community,” the group of species found in a particular location or habitat.
species concepts
The precise sets of criteria that systematists use when deciding whether two populations are members of the same species, or two separate species. Different species concepts tend to emphasize different such criteria, sometimes leading to disagreements about classification.
species population
A population of all the living individuals of a particular species; also called simply a species.
species richness
The number of species living in a location, ecological community, or region. The more species, the greater the species richness.
species-area plot
Usually a graph showing the cumulative number of species detected as the area sampled increases. The resulting relationship is typically a curve, as fewer and fewer new species are detected with increasing area.
species-area relationship
The nearly universal tendency for larger areas to harbor greater numbers of species; used most often in reference to islands or habitat fragments.
A graphical representation of a recorded sound (like a birdsong) that includes information on both frequency and loudness, usually by plotting frequency against time and indicating intensity by shading or colors.
See spermatozoa.
sperm competition
In birds, a form of intrasexual selection that arises after more than one male has copulated with the same female, and therefore the sperm of multiple males are in competition to fertilize an egg within the female’s reproductive tract.
spermatozoa (singular, spermatozoan)
Sperm cells or sperm; each spermatozoan is a single cell composed of a DNA-containing head and a propulsive tail. Sperm are short and simple in nonpasserines, and longer and spiral-shaped in passerines.
spinal cord
A cable of neurons conducting nerve impulses to and from the brain; it runs inside the vertebral canal of the vertebral column, extending from the medulla oblongata of the brain to the tail.
spinal nerves
Nerves that arise from the spinal cord in pairs—one on each side of the cord—all along the spinal cord; each pair carries sensory input and motor output, and serves a very specific region of the skin, or very specific muscles or other organs.
spinous process
A ridge of bone projecting from the dorsal surface of each vertebra; it is particularly well developed in the thoracic vertebrae.
Informal term for changes in classification that involve separating into different species populations that were previously considered to be part of a single species.
spotting scope
A medium-range telescope commonly used for bird watching, usually with magnification power between 15x and 60x.
spread-wing posture
A stance in which a bird stands motionless with the wings extended to the side, either to dry the wings or to absorb sunlight. Adopted by many large birds, such as cormorants, anhingas, pelicans, storks, and New World vultures.
Bony outgrowths from any part of the skeleton, such as the wing (see wing spurs).
square tail
Tail shape in which the rectrices are all about the same length.
stabilizing selection
Natural selection that acts against any change in a current characteristic. Stabilizing selection occurs when an organism is already very well adapted to current conditions, and it tends to keep the species at an optimal middle point.
stable age distribution
Property of some populations such that the relative proportions of individuals of different ages are the same each year, when the same time of year (such as the start of the breeding season) is compared. Note, however, that in a stable age distribution these proportions still change predictably throughout each year
staging areas
Locations where migrating birds fuel-up just before or during migration.
star compass
The mechanism by which nocturnally migrating birds are able to use the star patterns surrounding the North Star (in the Northern Hemisphere) to determine which way is north (and thus the other compass directions as well). A young bird initially learns how to use the star patterns by observing their rotation and using the point at the center of their rotation (the North Star, Polaris) as the direction to North; but once a bird learns the star patterns, it can find north even in a stationary planetarium.
statant cup nest
Cup nest constructed on top of a hard physical support or supports, including the ground, building ledges, or tree branches; built by American Robins, Horned Larks, and many other birds.
static pressure
The force, produced by random motion of molecules, that air exerts uniformly in all directions. When you squeeze a balloon, you feel the static pressure from the air inside.
static soaring
A type of soaring (flying without flapping or losing altitude) in which birds take advantage of the energy in rising air masses (either in thermals, called thermal soaring, or along hills or ridges, called slope soaring) to obtain lift with little or no energy expenditure on their part. Performed by birds with slotted high-lift wings.
statistical testing
Mathematical procedures used by scientists to determine whether a set of observations or the results of an experiment either (1) support their hypothesis, or (2) could result from chance alone. Most statistical tests generate a number that indicates the probability that the observations or results could have occurred from chance alone. The lower the probability, the greater the statistical significance of the data. Many scientists choose a 1 in 20 probability (.05) as the cut-off for significance
statoacoustic nerve
See vestibulocochlear nerve.
Dense crystals of calcium carbonate embedded, along with sensory hair cells, in a gelatinous material inside the utriculus and sacculus of the inner ear. As the head’s orientation changes with respect to gravity, the crystals settle against different hair cells, stimulating them; the brain determines the head’s position by detecting which hair cells are stimulated.
stepping stones
In conservation biology, small pieces of habitat, suitable for a particular species, that are located between larger habitat preserves; they allow individuals to move between the larger preserves.
steppingstone dispersal
Dispersal that proceeds by a series of colonizations, one after another, across a chain of islands or among habitat patches.
sternal rib
The ventral portion of each thoracic rib; it articulates with the sternum ventrally, and with the vertebral rib dorsally.
The breastbone; it provides an attachment site for the ribs and pectoral muscles. In all flying birds except tinamous, the sternum has a keel (carina).
In biology, describes events whose occurrence and/or outcomes are the result of random (or at least unpredictable) factors. For comparison, see deterministic.
stochastic extinction
Extinction resulting from random changes in population size when the population drifts to zero individuals.
Saclike, expandable digestive organ between the esophagus and the small intestine; in birds, the stomach consists of two parts
A foraging technique used by falcons in which a bird drops through the air at great speed in pursuit of a flying bird or insect.
stopover sites
Locations where migrating birds take a short break to rest, eat and drink for maintenance, and/or await favorable migratory conditions.
stress response
In avian endocrinology, the rise and fall in hormone levels (particularly corticosterone) during and after a bird is exposed to some kind of stressor.
structural colors
Colors not produced by pigments, but instead by the reflection of certain wavelengths of light off the actual physical structure of the object (such as a feather of a bird or a scale of a butterfly wing).
A collection of information on the individuals of a particular species. Key to a studbook is data on the relatedness among individuals—for example, which individuals are the parents, siblings, and offspring of a given individual. But studbooks may contain a variety of other information as well, such as data on each individual’s disease history, birth location and date, and location in the wild or in captivity. Studbooks are used primarily by researchers who are breeding endangered or declining species; the studbook information helps the researchers to pair individuals with the partners to whom they are least related, thus promoting and preserving as much genetic diversity as possible.
A bird that has not yet reached maturity; immature.
subadult plumage
Any of the plumages worn by young birds before they reach their definitive plumages (those of a mature bird).
subclavian arteries (left and right)
Arteries that carry blood to the front limbs (wings).
subclavian veins (left and right)
Paired veins that carry blood from the front limbs (wings); on each side, they merge with the jugular and pectoral veins to form the cranial vena cava, which returns blood to the heart.
Describes an egg that is longer than an elliptical egg, with a slight bulge to the sides.
suboscine passerines
The group within the order Passeriformes in which song learning is rare or absent.
Members of Suborder Tyranni, one of the two large suborders of Order Passeriformes (perching birds). The suboscines have less complex voice boxes than members of the other passerine order, Passeri, and thus must sing simpler songs. Includes birds such as tyrant flycatchers, antbirds, manakins, and cotingas.
Practice singing (somewhat like human infant babbling) that begins shortly after songbirds leave the nest. Subsong usually is quiet, garbled, and rambling compared to adult song; it contains some of the same elements, but they are strung together in odd ways. It gradually comes to resemble adult song.
A subset of a species, usually in a particular geographic area, that contains individuals that are morphologically distinct from other individuals of the same species, but are still capable of interbreeding with those other individuals. Also called a race.
A family (Promeropidae, 2 species) of long-billed, long-tailed, nectar feeders that specialize on Protea plants on the mountainous slopes of South Africa.
sun compass
A mechanism by which birds and some other animals can use the position of the sun in the sky to indicate compass direction. To do this, they must be able to compensate for the changing position of the sun during the day.
A family (Nectariniidae, 124 species) of tiny birds with long, slender, down-curved bills; they feed on nectar and insects and serve as pollinators of flowers. Sunbirds are similar to hummingbirds, but do not hover while feeding. Many males are brilliantly colored. Most species inhabit Africa, but some are found in tropical parts of the other Old World regions.
Exposing the plumage to the sun, usually by spreading the wings and/or tail (often against the ground), fluffing the feathers, and remaining motionless; sunning may aid Vitamin D production and remove ectoparasites, as well as warm the bird.
superciliary line
See eyebrow stripe.
supertramp species
A term applied to bird species that are particularly good at dispersing long distances and at colonizing newly created habitats.
supplemental plumage
In the Humphrey-Parkes system of nomenclature, a plumage that occurs in a few species, such as ptarmigans and certain buntings, in addition to the basic and alternate plumages.
supracoracoid foramen
See foramen triosseum.
The powerful flight muscle (also called the supracoracoid muscle) that raises a bird’s wing; it also slows down the wing at the end of the downstroke and accelerates it at the beginning of the upstroke.
surface diving
Diving under water from a swimming position on the water’s surface; performed by birds such as diving ducks, grebes, cormorants, and loons.
surface-to-volume ratio
An individual’s surface area (through which heat is lost) divided by its volume (which generates heat); the larger the ratio, the faster an individual will lost heat. Smaller birds have larger ratios than larger birds, and thus to compensate they must have higher metabolic rates and must consume relatively more food. Also called surface area-to-volume ratio.
In conservation biology, a stand-in for another individual or species. For example, researchers practiced releasing techniques for the highly endangered California Condor by first using the biologically similar but less endangered Andean Condor as a surrogate.
survival rate
The proportion of individuals in a population that survive for a particular interval of time—usually a year.
The proportion of individuals in a cohort (a number of individuals born during the same period of time) that survive to a given age.
survivorship curve
A graph of the proportions of individuals in a cohort (a number of individuals born during the same period of time) that remain alive at different ages. The survivorship curve shows the chance of mortality at different stages in an organism’s lifetime; the basic shape of the curve for a particular species or group of species illustrates the general pattern of mortality.
suture zone
The somewhat controversial idea that broad scale changes in past habitats have affected many species in similar ways and thereby created locations where multiple hybrid zones overlap.
Boundary lines along the junction between two bones; sutures are visible in the skull of a young bird.
A foraging technique in which a bird pursues insects in flight, capturing them in midair in its mouth; used by swifts, nighthawks, and many swallows, among others.
In a bird song, a note or cluster of notes that forms a unit that is repeated.
An ecological relationship between organisms of two different species that live in close association with one another; types of symbioses include mutualism, commensalism, and parasitism (see separate entries for each).
sympathetic ganglion
A clump of cell bodies of sympathetic neurons that arise in the spinal cord. A chain of sympathetic ganglia runs the length of the spinal cord near its ventral surface.
sympathetic system
The part of the autonomic nervous system that functions under conditions of stress; the sympathetic system prepares the body for “fight or flight” by increasing the heart rate, breathing rate, and blood pressure. It consists of nerves that leave the spinal cord from the thoracic and lumbar regions.
sympatric speciation
Speciation that occurs even when the two diverging populations breed in the same locations and therefore have the possibility of intermixing within each generation.
Refers to organisms or groups of organisms that occur in the same place.
A small gap between nerve cells, across which nerve impulses travel.
synchronous hatching
Pattern of hatching in which all the eggs of a single clutch hatch at about the same time (on the same day), resulting in a brood of young all the same size and age. This hatching pattern occurs when incubation is delayed until the last egg is laid. Because development of laid eggs does not begin until they are warmed, all the embryos begin to develop at the same time and are ready to hatch at the same time.
syndactyl feet
Foot arrangement in which the hallux points backward and toes two, three, and four point forward, with toes two and three (the inner and middle toes) fused for much of their length. Found in many kingfishers and hornbills.
The segment of the vertebral column of birds that is formed by the fusion of some thoracic vertebrae with all of the lumbar, all of the sacral, and the first few caudal vertebrae; the synsacrum is in turn fused on either side with the ilium bones of the pelvis.
syringeal muscles
The muscles of the syrinx; they allow the syrinx to change shape and thus to produce different types of sounds.
The primary sound-producing organ of birds; located at the point where the trachea divides to form the two bronchi.
systemic circulation
Portion of the circulatory system that carries blood from the heart to the body tissues (excluding the lungs) and back to the heart.


See boreal forest.
tail bone
See pygostyle.
A Neotropical suboscine family (Rhinocryptidae, 52 species) related to antbirds; tapaculos have cocked tails and frequent more southerly, open, and dry areas than do antbirds.
Ankle bones. In humans, they remain distinct; in birds, the proximal tarsals fuse with the tibia to form the tibiotarsus, and the distal tarsals fuse with the metatarsals (instep bones) to form the tarsometatarsus.
The long bone supporting the upper section of the bird foot; the tarsometatarsus is formed by a fusion of the distal tarsals (ankle bones) and the metatarsals (instep bones). Also called simply the metatarsus.
The upper section of the avian foot, between the heel and the toes.
taste buds
Simple structures, usually embedded in the epithelium of the oral cavity and the tongue, that are the receptors for taste sensations in all vertebrates. Humans have numerous taste buds located on the tongue, but birds have few taste buds, which are located primarily on the roof of the mouth or deep in the oral cavity, with none on the tongue.
The classification of organisms—assigning names and relationships.
tectorial membrane
The upper membrane of the cochlear duct in the inner ear. Sound waves set the lower membrane—called the basilar papilla in birds—into motion, moving its hair cells against the tectorial membrane and triggering nerve impulses in the hair cells, which are sent to the brain in the process of sound perception.
The anterior portion of the forebrain; it contains the olfactory lobes.
Moderate; describes a climate free from extreme heat and cold, but experiencing some of both; generally found in the middle latitudes.
temporal (or allochronic) speciation
An unusual form of speciation in which the two diverging populations occur in the same location, but diverge because they reproduce at different times of the year.
temporal fovea
Area in the posterior quadrant of the retina of hawks and other fast-flying diurnal avian predators where the cones are most concentrated and the neural layer (the nerves from the rods and cones, which overlie the rods and cones and block some light) is the thinnest, and thus vision is the sharpest. The temporal foveae provide sharp binocular views of the area in front of the bird.
A band of fibrous connective tissue that attaches muscles to bones, and/or binds many muscle fibers together to form a skeletal muscle.
Giant, soaring, scavenging birds of the New World (subfamily Teratornithinae) whose fossils date from the late Tertiary period; teratorns became extinct around 10,000 years ago when many large mammals were wiped out. The largest species had a wingspan approaching 19.4 feet (5.9 m).
An active termite nest.
A behavior in which an individual defends an area against other individuals.
A defended area.
tertiary consumers
Level in a food chain or web that consists of organisms—usually animals—that directly eat secondary consumers.
testes (singular, testis)
The male gonads; one testis lies at the cranial end of each kidney. The testes produce sperm as well as the male sex hormone, testosterone, and in some species, a female sex hormone, estrogen.
A diverse group of reptiles from the early Mesozoic, all of which have a diapsid skull, teeth set in sockets, and an antorbital fenestra (an opening on each side of the skull in front of the eye socket). Also known as basal archosaurs or pseudosuchian thecodonts.
Theory of Island Biogeography
A relationship formalized by ecologists Robert MacArthur and E. O. Wilson that describes how the total number of species inhabiting an island or habitat patch results from a balance between the rate at which new species colonize and the rate at which populations of established species become extinct.
A rising column or large bubble of warm air that results from differential heating of land surfaces by the sun; thermals develop over areas that are darker than surrounding areas, and over south-facing slopes.
thermal neutral zone
The range of ambient temperatures over which a bird does not require additional metabolic energy for the heating or cooling of its body.
thermal soaring
A type of static soaring (see separate entry) in which birds use the rising air in thermals (rising columns of warm air) to propel themselves upward, circling higher and higher with little energy expenditure of their own. Once high in the air, they can glide out of the thermal and across the countryside in whatever direction they wish to travel.
The active ability of an organism to modulate its internal body temperature.
The upper leg, supported by the femur.
thoracic air sacs
Two pairs of air sacs in the chest region of birds, one pair (a cranial and caudal sac) located on each side of the body.
thoracic cage
The rib cage; it consists of the ribs connected to the thoracic vertebrae above and to the sternum (breastbone) below. It forms a flexible but strong enclosure for the heart, liver, and lungs, as well as for the thoracic air sacs of birds.
thoracic ducts
Paired lymph ducts that collect the products of fat digestion from the intestinal lymph trunk (coming from the small intestine; see separate entry); the thoracic ducts run along the surface of the aorta and eventually deliver their contents to the venous system at the cranial vena cavae just before these large veins enter the heart.
thoracic vertebrae
The vertebrae of the thorax (chest) region; they articulate with the ribs and thus form part of the rib cage. In birds, some thoracic vertebrae are fused with lumbar, sacral, and some caudal vertebrae to form the synsacrum.
Nucleated blood cells that resemble red blood cells, but are more dense and complex and are highly specialized to carry out blood clotting. They serve the same function that platelets serve in mammals, which do not have thrombocytes.
In a bird, the portion of the force generated by the flapping wings that propels the bird forward (in the direction the bird is moving, which is always the direction opposite to drag).
thyroid glands
Paired endocrine glands at the base of the neck. Under the control of the anterior pituitary, the thyroid glands secrete the hormones thyroxin and triiodothyronine, which regulate the annual increase in gonad size, sperm and egg production, the growth and pigmentation of feathers, and molting.
In humans, the larger of the two lower leg bones; in birds, the tibia fuses with several tarsal (ankle) bones to form the tibiotarsus, which supports the crus.
Bone supporting the lower leg (crus) of birds; the tibiotarsus is formed by the fusion of the tibia with the proximal tarsal (ankle) bones.
See oxpeckers.
time compensation
In the course of orientation or navigation, making allowances for the changes in the position of the sun (or other celestial bodies) in the sky throughout the day.
Primitive, grouse-like birds (family Tinamidae, 46 species) whose eerie calls haunt both forests and pampas of South and Central America; tinamous are the only ratites capable of flight.
tip vortex
A type of turbulence created at the tip of a wing; it consists of air currents spiraling off the wing tip behind a bird or other flying object. These eddies sometimes may be seen as trails of white behind the wing tips of large airplanes as they take off or land.
An aggregation of cells with related and often very similar characteristics; examples include muscle tissue and bone.
tissue fluid
Blood plasma that has moved out of blood cells at the capillary beds, and is in the minute spaces between the body cells. Oxygen and nutrients diffuse out of it to the body cells, and carbon dioxide and other wastes diffuse in. Eventually, tissue fluid returns to the blood system, either by diffusing directly back into a blood capillary near the venous side, or by diffusing into a lymphatic capillary (in which it is called lymph), which carries it to larger lymph vessels and eventually into the venous system.
tongue bone
See entoglossal.
top-down effect
In ecology, a situation in which the amount of predation or consumption by organisms at the top of the food chain have large effects on lower components of the ecosystem.
A profound state of sleep, in which the body temperature drops and consequently all metabolic processes and stimulus-reaction processes slow down. Used by swifts, hummingbirds, nighthawks, and some other birds to conserve energy when food resources are unavailable. Birds may enter torpor on a nightly basis, or for extended periods up to an entire season of cold weather. Also called hibernation.
total fitness
The sum of direct fitness via the individual’s own reproduction, plus their indirect fitness via their relative’s reproduction. Sometimes also termed “absolute fitness.”
The windpipe; the tube conducting air from the larynx to the lungs.
tracheal bulla
An expanded sac on one side of the lower end of the trachea in males of many duck species; it is thought to modify the sounds produced by the syrinx.
A flying bird’s actual direction of movement with respect to the ground; due to crosswinds, a bird’s track may not be the same as its heading. Also applies to aircraft and other flying organisms.
Bundles of axons and their myelin sheaths within the central nervous system; collectively, tracts form the white matter, found in the outer portions of the spinal cord and the inner areas of the brain.
tragedy of the commons
A common dilemma in natural resource management, in which benefits to an individual are contrary to the collective benefits of the group, so that individuals are inclined to act selfishly, despite this being to the detriment of everyone. Named after communal grazing lands (the “commons”) to which each herder could freely add additional livestock, thereby helping themselves in the short term but ultimately leading to severe overgrazing and the destruction of the commons as a long-term resource for the community.
transect count
A bird census technique in which an observer moves along a path—the transect—and notes all birds seen or heard.
transequatorial migrants
Birds that migrate back and forth across the equator to take advantage of spring and summer in both hemispheres.
transitive inference
The ability to understand and infer linear relationships among individuals. For example, the ability to understand that “if A is dominant to B, and B is dominant to me, then A is dominant to me.”
In conservation, the human-assisted movement of individuals from one location to another, used most effectively for areas where threats have been reduced or eliminated.
transverse plane
A vertical plane through an organism, dividing the body into cranial and caudal portions.
Foraging movements in which a bird rotates through a set of discrete food sources; most often applied to individual hummingbirds that visit a set flower patches in an established and repeated sequence, allowing the time in between for the flowers to replenish their nectar.
trees-down theory (of the origin of avian flight)
See arboreal theory.
trial-and-error learning
Learning to associate one’s own behaviors with either a reward or punishment; behaviors that result in reward are repeated, and those that result in punishment are abandoned.
trigeminal nerve
The fifth cranial nerve; it divides into the ophthalmic, maxillary, and mandibular nerves (see separate entries) after exiting the brain.
A phrase or song that consists of a single note or cluster of notes (a syllable) that is repeated many times in rapid succession.
triosseal canal
See foramen triosseum.
Trivers-Willard hypothesis
The idea that females should produce offspring of the high investment sex when the females are in good condition, and offspring of the lower investment sex when the females are in poor condition.
trochlear nerve
The fourth cranial nerve; it carries motor output from the brain to a single eye muscle.
Pertaining to feeding.
trophic cascade
A situation in which predators high in a food chain exert top-down effects that ramify through the food chain levels to influence the abundance of species that the predators do not eat directly.
trophic levels
The different levels of food production or consumption within a food chain or web; for example, producers, primary consumers, secondary consumers, and decomposers.
trophic structure
Organizational scheme of a community that is based on the feeding relationships between organisms
Tropical Marine region
The major faunal region of the seas that includes the warm equatorial waters between the Subtropical Convergences of both hemispheres.
true navigation
The ability to take the proper course toward a specific goal, even from completely unfamiliar sites at vast distances from known areas.
A family (Psophiidae, 3 species) of large, hump-backed, chicken-like birds that inhabit the rain forests and floodplains of Amazonia. Trumpeters forage on the ground in flocks, but roost and nest colonially in trees. They eat both plants and animals (including reptiles and amphibians, and even carrion), and sometimes follow army ants for the arthropod prey they disturb.
Birds in the order Procellariiformes, including albatrosses, shearwaters, fulmars, petrels, and storm-petrels, all of which have tubular nares (see separate entry).
tubular nares
Nares (nostrils) that open into a horny, tubular structure that sits on top of the bill; in some species the tube for each nostril is separate, but in others the two adjacent tubes fuse to form a double tube—within which the tubes coming from each nostril may remain separate or may be partially fused. Tubular nares are found in the Procellariiformes, an order of pelagic seabirds that excrete large amounts of salty fluid through their nostrils. The tube is thought to reduce heat and airflow at the nostril (where the nasal cavity opens into the upper beak), allowing the salty fluid to flow down the beak and away from the nostril before evaporating. Thus the salts remaining after evaporation are less likely to clog the nostril and salt gland than if evaporation occurred right at the nostril.
A porous, white limestone formed under water when springs or rainwater carry dissolved calcium into lakes that have a high concentration of bicarbonate ions (found in baking soda). When the two solutions mix they form calcium carbonate, which precipitates (comes out of solution), creating tufa—which often accumulates into underwater towers or crusts covering the lake bottom. Mono Lake, in California, is well known for its tufa formations, which became exposed when the lake’s water level dropped dramatically after water was diverted for use in the Los Angeles area.
See arctic tundra or alpine tundra.
African relatives of cuckoos, turacos are noisy and arboreal, and tend to run squirrel-like along branches. The plumage of turacos is soft green (one of the few green pigments known from birds), blue, or gray, often with bright red (or in a few species, purple) on the wings. The 18 species of turacos, together with plantain-eaters and go-away-birds, make up the family Musophagidae.
A disorderly flow of air; turbulence may interfere with smooth airflow over a bird’s wings and thus may disrupt flight.
tutor song
A song played to a bird in the laboratory during experiments on song learning.
tympanic canal
The lower perilymph-filled canal inside the bony cochlea of the inner ear; the tympanic canal is connected to the (upper) vestibular canal at one end. Pressure waves in the perilymph of the two canals are transmitted to the endolymph of the cochlear duct between them, setting the (in birds) basilar papilla in motion and stimulating the sensory hair cells.
tympanic membrane
A membrane that stretches tightly across the external ear canal; the tympanic membrane vibrates when struck by the pressure waves of a sound, and transmits those vibrations to the (in birds) columella. Also called the eardrum.
tympaniform membranes
Flexible membranes that stretch between successive cartilaginous rings of the syrinx, allowing the syrinx to change shape (in response to contractions of the syringeal muscles) and thus to produce different types of sounds.
One of the two large suborders of Order Passeriformes (perching birds). The Tyranni (also called suboscines) have less complex voice boxes and thus sing simpler songs than members of the other passerine order, Passeri (also called oscines). Suborder Tyranni includes birds such as tyrant flycatchers, antbirds, woodcreepers, ovenbirds, tapaculos, manakins, and cotingas.
Extremely diverse New World suboscine family containing nearly 400 species; members of the family Tyrannidae are commonly known as tyrant flycatchers. No other bird family has more species.
tyrant flycatchers
The nearly 400 species of the diverse New World suboscine family Tyrannidae.


The larger and thicker of the two bones that make up the antebrachium.
The larger of the two wrist bones of birds; the ulnare consists of several fused carpals.
In biology, pertaining to long-term causes; evolutionary. Ultimate questions about a behavior, for example, look into the factors that might have caused the particular behavior to evolve—why it evolved. For comparison, see proximate.
ultimate factors
In biology, the variables affecting the survival and reproduction of a population and its ancestors, and therefore influencing the evolution of certain traits, behaviors, or physiological processes, or the timing of such things. For example, the food supply at different times of the year is a major ultimate factor that affects the timing of breeding in birds. For comparison, see proximate factors.
ultimate questions
In biology, questions that pertain to long-term evolutionary processes, often addressing “why” a particular trait arose within a species or higher taxonomic group.
ultimobranchial glands
Two small, light-colored glands located near the parathyroid glands, usually on an artery; the ultimobranchial glands secrete the protein calcitonin, which lowers the blood calcium concentration.
umbrella species
A species for which successful conservation actions would also help preserve many other species typical of the biological community in which it lives.
uncinate process
A flattened, hook-shaped extension of bone that projects caudally from the vertebral segment of each rib. Because each uncinate process overlaps with the rib behind it, the processes help to strengthen the rib cage. Found only in birds.
understory vegetation
The shrubs and shorter trees in a forest; the understory vegetation grows below the level of the main canopy and mid-story vegetation (see separate entry), and above the ground vegetation.
undertail coverts
The short contour feathers that cover the bases of the rectrices on the ventral side of the tail.
underwing coverts
The feathers lining the underside of the wing, from the leading edge of the wing to the flight feathers.
upper beak
The upper half of the beak; also called the upper jaw.
upper jaw
See upper beak.
uppertail coverts
The short contour feathers that cover the bases of the rectrices on the dorsal side of the tail.
A process by which cold, nutrient-rich water from the lower ocean depths rises to the surface. Upwelling may occur where certain ocean currents meet, or where winds blowing away from the coast move the surface water away from shore, causing water from below to rise to replace it.
Two tubes, one leading from each kidney, that carry urine from the kidneys to the (in birds) cloaca.
uric acid
A complex but less toxic molecule to which the liver of birds converts the highly toxic nitrogenous waste products of protein metabolism. Compared to urea, the nitrogenous waste product of mammals, uric acid requires more energy to produce, but it is less soluble in water and more highly concentrated, allowing birds to excrete more toxic waste per molecule excreted, while minimizing water loss.
urinary bladder
Sac that stores urine before it is excreted; among birds, the urinary bladder is only present in the South American rheas.
urinary system
See excretory system.
urogenital system
The combination of two organ systems: the urinary (excretory) system, which removes toxic nitrogenous wastes from the blood, and the genital (reproductive) system. The two systems often are considered together because they are located close together in the body, and because they may share some structures (such as the cloaca, in birds).
uropygial gland
A gland near the upper base of the tail that secretes oils that a bird distributes across its feathers via preening. Also known as the oil gland.
General term (used primarily by the Germans) that refers to early, primitive birds such as Archaeopteryx.
One of the two membranous chambers inside the bony vestibule of the inner ear (the other is the sacculus); these chambers contain hair cells and dense crystals called statoconia, both of which are embedded in a gelatinous material that is surrounded by endolymph. The statoconia and hair cells perceive the position of the head with respect to gravity (see entry for statoconia).


In birds, the short, lower section of the oviduct, after the shell gland; the vagina opens into the left side of the cloaca.
Birds that move far beyond the normal range of their species.
vagus nerve
The tenth cranial nerve; it carries sensory and motor information between the brain and the pharynx, larynx, heart, lungs, gizzard, liver, and intestines.
The broad, flat surface on each side of the shaft of a contour feather; the vanes are formed by the interlocking of adjacent barbs.
A diverse songbird family (Vangidae, 14 species) of shrike-like birds. Most vanga species are gregarious and noisy, gleaning insects and other small animals as they move through the trees in groups.
vasa deferentia
See deferent ducts.
A quantity with both magnitude (size) and direction; for example, a bird’s flight at 25 miles per hour to the south is considered a vector. A vector is often shown as an arrow whose length represents magnitude and whose orientation indicates compass direction.
An organism (usually an insect, tick, or other arthropod) that transmits a pathogen from one kind of host to another.
vector navigation
Finding one’s way by adding vectors composed of compass direction and distance in order to determine the straight-line path to a desired destination.
A vessel that carries blood toward the heart. All veins except the pulmonary vein and its tributaries carry oxygen-poor blood.
vena cava
See caval veins or cranial vena cavae or caudal vena cava.
venous ring
A ring of veins formed by the left and right renal portal veins of birds; the venous ring connects the various lobes of the two kidneys, and is part of the avian renal portal system.
The opening of the cloaca to the exterior of the body. In birds, the vent is the only posterior opening, and thus it releases feces from the digestive system, uric acid from the excretory system, and sperm or eggs from the reproductive system.
Toward the belly of an organism.
The two thick-walled, muscular, posterior chambers of the heart; the ventricles receive blood from the atria and then pump it to other parts of the body
Small veins that carry blood from the capillaries to larger veins.
vertebrae (singular, vertebra)
The individual bones that form the vertebral column (backbone).
vertebral canal
A long tube that runs the length of the back, inside the backbone; the vertebral canal is formed by the vertebral arches of successive vertebrae. The vertebral canal protects the spinal cord, which runs inside it from the brain to the tail.
vertebral column
Commonly called the backbone or spine, the vertebral column is a series of complex, uniquely articulating or rigidly fused vertebrae that provide support for the head, neck, ribs, back, and tail.
vertebral rib
The dorsal portion of each thoracic rib; the vertebral rib articulates with the thoracic vertebra dorsally and with the sternal rib ventrally.
Animals that have a backbone
vertical migration
Type of migration in which animals move seasonally up and down mountainsides.
vestibular canal
The upper perilymph-filled canal inside the bony cochlea of the inner ear; the vestibular canal is connected to the lower perilymph-filled canal, the tympanic canal, at one end. Pressure waves in the perilymph of the two canals are transmitted to the endolymph of the cochlear duct between them, setting the (in birds) basilar papilla in motion and stimulating the sensory hair cells.
vestibular window
A soft, oval region on the bony cochlea, to which (in birds) the columella of the middle ear is attached. Movement of the columella moves the vestibular window, sending pressure waves through the fluid (perilymph) inside the cochlea. Formerly called the oval window.
Bony structure of the inner ear containing two membranous chambers, the utriculus and sacculus, which contain hair cells that perceive the position of the head with respect to gravity. The vestibule is part of the bony labyrinth of the inner ear and thus contains perilymph.
vestibulocochlear nerve
The eighth cranial nerve; the vestibulocochlear nerve carries sensory input for balance and hearing. Its vestibular (balance) portion carries sensory input from the semicircular canals and vestibule of the inner ear, and its cochlear (hearing) portion carries sensations from the hair cells of the inner ear. Formerly called the auditory, acoustic, or statoacoustic nerve.
Describes a structure with little current apparent function that is thought to be present solely because it derives from a structure that was functional in an ancestor.
vestigial structure
A structure that has little or no apparent function and is thought to be present in an organism only because it has been inherited from an ancestor. Many vestigial structures are reduced in size from the ancestral (and functional) condition, and presumably are in the process of being eliminated through evolution. A vestigial structure in humans is the remains of a nictitating membrane in the inner corner of each eye—in reptiles, birds, and some other mammals the nictitating membrane is functional (see separate entry).
The subdivision of a species’ originally large distribution into smaller and separated geographic fragments via some new geographic barrier or the fragmentation of the species’ originally widespread habitat.
vicariant speciation
Speciation that results over time after populations that become divided by new barriers have diverged from one another.
Refers to members of the African genus Vidua, which consists of whydahs and indigobirds. Viduines are colorful seed-eaters that are brood parasites on other members of their family, Estrildidae; they are known for the intricate patterns inside the mouths of their nestlings, which strikingly resemble those of their host’s nestlings.
villi (singular, villus)
Minute fingerlike projections. Villi often are produced by folds in a tissue lining an organ—as in the inner surface of the small intestine of vertebrates, where the presence of villi greatly increases the surface area available for absorbing nutrients.
visual field
See field of view.
vitelline diverticulum
Small pouch of tissue at the junction of the jejunum and ileum of the small intestines of many birds; the vitelline diverticulum is a remnant of the yolk sac of the embryo.
vitelline membrane
In avian biology, the transparent membrane surrounding and holding together the yolk of an egg.
vitreous body
Clear, gelatinous material filling the vitreous chamber (see separate entry) inside the eyeball. The vitreous body gives rigidity to the eyeball and helps to maintain its shape.
vitreous chamber
The largest chamber inside the eyeball; the vitreous chamber is located on the posterior side of the eyeball and is filled with a clear, gelatinous material called the vitreous body (see separate entry), which gives rigidity to the eyeball.
vocal repertoire
All the different types of vocalizations that are produced by an individual bird, including both songs and calls; for comparison, see song repertoire.
vocal signals
The collection of songs, notes, and calls that birds use to communicate.
voluntary muscles
See skeletal muscles.


warm front
The interface between a warm air mass and the cold air mass it is overtaking; the warm air tends to push up over the denser cold air and become cooler, which results in the formation of clouds and eventually precipitation.
See endothermic.
water birds
See aquatic birds.
Ducks, geese, and swans; family Anatidae.
Common name for members of the family Callaeidae, which consists of two living species (the Saddleback and Kokako) and one extinct species (the Huia). Wattlebirds are forest-dwelling songbirds endemic to New Zealand; they are named for the pair of colorful, fleshy wattles at the corners of their mouths.
Ornamental flaps or folds of skin that dangle from the head or neck of some birds. Wattles are found in turkeys, pheasants, cotingas, wattlebirds, and many others.
Together with whydahs, indigobirds, and numerous related species, these colorful seed-eaters of the Old World form the large family Estrildidae (159 species). The Zebra Finch, a well-known cage bird, is a waxbill.
A diverse Old World family (Ploceidae, 117 species) of colorful seed-eaters; weavers are found primarily in open tropical areas and are named for the ability of many species to weave large, complex nests.
See sap well.
western montane forest
North American coniferous forest ecosystem that is dominated by spruce and fir trees and is found along the North Pacific slope and in the Cascades, Sierra Nevada, and Rocky Mountains; western montane forest hosts few breeding birds.
wetland mitigation
Compensation that the recipient of a permit to fill or otherwise destroy wetland areas is legally required to provide, to make up for the wetland area lost. Compensation may consist of restoring or re-creating other wetland areas of equal or greater size compared to the wetland area destroyed, or may be a cash payment into a mitigation bank—a fund managed by a public agency or private conservation group that carries out large-scale wetland restoration projects using money accumulated from a large number of small-impact projects.
whisker stripe
A distinctively colored stripe in the malar region of birds; also called a mustache stripe or malar stripe.
A song or phrase in which clear tones are produced one pitch at a time.
Large, round-headed songbirds with slightly hooked bills; whistlers are known for their explosive, often-beautiful, songs. The nearly 40 species of whistlers, along with shrike-thrushes, pitohuis, and a few related species, form the family Pachycephalidae, which is found primarily in the Australasian region.
white blood cells
Unpigmented blood cells of several different types, all of which contain a nucleus and can leave the capillaries and move about in the spaces between the body cells. Many types of white blood cells are important in fighting infections. Also called leukocytes.
white fibers
Muscle fibers that appear lighter in color than red fibers because they have fewer capillaries (and thus fewer red blood cells) and less of the reddish oxygen-transport molecule myoglobin; white fibers also are larger in diameter than red fibers. Muscles with many white fibers are used for quick bursts of action, but they cannot carry out sustained activity because they produce energy anaerobically (without oxygen)—so lactic acid builds up quickly and causes fatigue. Muscles with many white fibers often are called “white meat” (as in the breast of a turkey).
white matter
Lighter-colored tissue (compared to gray matter) that makes up much of the brain and spinal cord. White matter consists of numerous tracts, which are bundles of nerve cell axons and their myelin sheaths, and is found primarily in the outer portions of the spinal cord and the inner areas of the brain.
white meat
See white fibers.
Members, along with indigobirds, of the African genus Vidua (members of this genus are called viduines). Viduines are colorful seed-eaters that are brood parasites on other members of their family, Estrildidae; they are known for the intricate patterns inside the mouths of their nestlings, which strikingly resemble those of their host’s nestlings.
In avian biology, a beakful of vegetation that is dipped in mud and then used to transport the mud to a nest that is under construction; usually the entire wick is added to the nest.
wing loading
The ratio of body weight to wing area; wing loading is a measure of how much “load” each unit area of wing must carry.
wing pouch
A pocket under each wing of male Sungrebes (Neotropical inhabitants of wooded streams); the wing pouch is formed from a pleat of skin and is used to carry the young while the male flies or swims.
wing spurs
Bony outgrowths of the carpometacarpus (the main bone of the avian manus); wing spurs are found in various birds such as cassowaries, plovers, sheathbills, screamers, and jacanas.
A foraging technique used by some herons, egrets, and storks in which a bird wading through the water quickly raises or brings forward one or both wings, apparently to frighten prey out of hiding or to provide a shady place where unsuspecting prey may try to hide, thus bringing the prey within the bird’s reach.
The distance between the tips of the fully extended wings of a living bird.
Part of a twilight territorial and courtship display given by both male and female Common Snipe. The birds circle high in the air and produce a series of rapid, pulsating, whistle-hums (called winnows) by spreading the tail and diving at high speeds—causing the stiff outer tail feathers to vibrate. European Snipe also winnow.
winter plumage
See basic plumage.
See furcula.
A Neotropical family (Dendrocolaptidae, 51 species) of similar-looking, rust-colored, suboscines with a wide array of beak shapes; woodcreepers typically forage in tree bark much like the unrelated Brown Creeper of North America.
A family (Phoeniculidae, 8 species) of sociable African birds with glossy, dark plumage and long tails; woodhoopoes nest in tree cavities and breed cooperatively—with additional adult birds helping the parents to tend the nest.
A family (Artamidae, 14 species) of small, chunky songbirds with a graceful, swallow-like flight; woodswallows inhabit the Australasian and Oriental regions and are known for huddling together on branches or in tree cavities in groups of up to 50 or more individuals.
A family (Parulidae, 115 species) of small, insectivorous birds, many of which are colorful, found in the New World. Many wood-warblers are Neotropical migrants. Also called New World warblers.



In avian biology, the familiar, yellow portion of an egg; the yolk contains nearly all of the lipids (fat) and most of the protein needed by the developing embryo. The yolk is surrounded and held together by the transparent vitelline membrane.
yolk sac
In avian biology, the extra-embryonic membrane inside the egg that surrounds the yolk of the developing embryo.
yolking up
The deposition of yolk within the vitelline membrane in alternating bands of darker and lighter yolk; yolking up is part of the egg-formation process in birds and occurs before egg laying.


Microscopic animals and protozoa that float freely in aquatic environments; zooplankton make up the “animal” portion of plankton.
A German term for the nocturnal restlessness that caged migratory birds exhibit when they are ready to migrate. Also known as migratory restlessness.
zygodactyl feet
Foot arrangement in which toes two and three point forward and the hallux and toe four point backward. Found in woodpeckers, cuckoos, toucans, owls, Osprey, turacos, most parrots, and some other birds.
The fertilized egg; the single-celled product resulting from the union of (1) the nucleus of the sperm cell from the male and (2) the nucleus of the ovum (egg cell) from the female. In birds, fertilization occurs in the infundibulum of the oviduct.