Thanks for the question. Corvids around the world vary in size, shape, and color. The Crow, Not Crow Snap ID isn't specifically designed to help you learn corvid vs. non corvid but it could help you somewhat.  This course is specifically about one species: American Crows. People shouldn't assume a fact about an American Crow would also be true about jackdaws, rooks, ravens, other crow species etc. For instance the breeding system varies from species to species. American Crows are socially monogamous and do cooperative breeding but other species of corvids that isn't often the case. Visually there are differences in ever species. Such as we are teaching about how the throat feathers on an American Crow differ from throat feathers on a Common Raven. You won't come across an American Crow in the UK so learning the differences between American Crow and Common Raven won't help you in UK but will help you if you visit USA.  Even in the USA American Crow differs from Fish Crow in various ways such as how the primary feathers appear flight. For visual identification of UK corvid species this won't help. It  might help if you were at a level of not knowing the difference between a starling and a corvid and a sparrow. American Crows are not the same species as a rook so there will be many differences, however not as many differences as between a rook and a sparrow or a rook and a starling.

Sound is certainly the best way to tell apart American Crows vs. Fish Crows as you have already pointed out. Even experts sometimes cannot tell these species apart visually unless they have them 'in the hand'. From the instructor: Fish Crows are a bit more glossy purple, but  this difference is very difficult to detect. Fish Crows have more slender bills and feet than American Crows. (Foot size is especially noticeable when contrasted against the large-footed south Florida race of the American Crow, C. b. pascuus.)  But bill size varies quite a bit in American Crows, with males having larger bills. A small billed female American Crow can have the same look in the face as a Fish Crow. Fish Crows show a shorter legged look when walking on the ground.  The "thigh" (actually the tibiotarsus) shows less prominently than is typical with an American Crow. The feathers on the upper back of American Crows show a decidedly "ringed" or "scaled" effect not shown by Fish Crows.  This character is especially obvious when the crow is between the sun and the observer. The upper back feathers of Fish Crows are more lax at the tips (without barbs holding them together), and consequently the upper back is without rings. In fact, on the upper back no individual feathers can be distinguished.  See if you can detect this difference on the head shots at the top of the page.  (The exposure of the shots is very different, but the effect is still there.) The "Wing Formula" differs between the species.  The outermost primaries on the wings are of different proportions.   On a bird in the hand these can be measured easily, but they are not easily seen in the field.  Primaries (the feathers that come out of the hand part of the wing) are numbered starting from the inside of the wing, and in crows the outermost one is primary number 10.  In Fish Crows, the 9th primary (p9) is the same size as, or is slightly longer than the 5th primary (p5), while in American Crows p5 is markedly longer than p9.   The effect of this differences is that the Fish Crow wing is somewhat more pointed in appearance. This difference is hard to see in the field and is not definitive.  It should be used as an indication only.  Molt and individual wear can give misleading impressions. Fish Crows have a faster wingbeat than American Crows.  In an area where both species occur the difference in wingbeat frequency can be an indication of the species, but this is not a definitive character.  Not all crows of a given species fly at the same wingbeat rate.  Juvenile and yearling American Crows usually fly with a faster beat rate than do adults (just not as efficient fliers?), and molt or feather wear can affect the rate too.  And sometimes crows are just more in a hurry than at other times. Fish Crows use a different posture for calling than American Crows. Fish Crows tend to stay more hunched and shorter necked, and typically fluff their throat feathers. American Crows tend to stretch their necks a bit, and do not substantially fluff their throat feathers. American Crows molt earlier than Fish Crows.  During the summer the two species can be distinguished by their stages of molt.  In much of their range American Crows breed a month or more earlier than Fish Crows (Johnston 1961, The biosystematics of American crows, Univ. of Washington Press; Clapp and Banks 1993, Raven 64: 90-98; McGowan 2001, Fish Crow (Corvus ossifragus) in Birds of North America, No. 589).   In upstate New York American Crows start incubation the first week of April, while Fish Crows don't start until May.  Most birds do not molt until they are finished breeding, and the molting schedules of the two crows reflect the month difference in their breeding schedule.  The earliest Ithaca American Crows (non-breeders and those whose nests have failed) will start molting in June and will be finished by the end of September.  Ithaca Fish Crows, on the other hand, start molting in late July and don't finish until October.  Source: https://www.birds.cornell.edu/crows/FishCrow.htm

FYI to everyone: In all our ID courses make sure to click "replay" on the Snap IDs as each time it will give you new match ups and pull in some photos you haven't seen yet. I suggest in any course that has a Snap ID play that one over and over as it helps to train your brain.