Greater black-backed and herring gulls breed on Appledore Island of the New England coast. Here the gulls court, mate, nest, and fledge young, all in three short months. But these two species are both predators, eating anything they can catch, kill, and tear apart, including other gulls. How can thousands of birds each a potential predator on the eggs and chicks of their neighbors coexist on a 95 acre island? The answer, these birds have evolved a complex system of social signals that keeps violence to a minimum. The gulls use these signals to carve out and hold small territories spaced just 4 to 5 meters apart across the rocky island. Territoriality begins when males arrive in spring followed closely by their mate returning to the same patch of rock they held in past years. These experienced pairs strut about. Inspecting the site for appropriate nesting spots and reenforcing territorial boundaries. Boundaries must be constantly defended. An interloper or neighbor approaching another’s territory is met with a series of signals or displays intended to drive him away. This is a long call. In a territorial context, a long call is a threat directed to trespassers. The mew, is also used in territorial disputes often calling in support from a mate. The Kek Kek is used when the colony is disturbed or in boundary disputes. The Yeow, serves these same two functions. This visual display is an Upright Posture. The Upright Posture is rigid, neck stretched up and forward, head pointed slightly down. Wings are cocked forward and slightly off the sides, poised to attack. Still more aggressive is the charged display. A ritualized attack that is amplified by out stretched wings, making the charging gull appear larger. Fights happen, in spit of the signaling system. But fights are risky as severe injury to a parent will doom their eggs and chicks. Over evolutionary time, selective pressure on individuals has favored communication over conflict resulting in a signaling system that reduces violence in the breeding colony.End of transcript
How can aggressive, predatory, and cannibalistic birds coexist in crowded breeding colonies? Explore the lives and territorial interactions of Herring and Great Black-backed gulls in a breeding colony on Maine’s Appledore Island and learn what signals gulls have evolved to prevent violent clashes.