In much of the country, winter birding means scanning open water in search of ducks, loons, cormorants, and anything else that might be out there. The birds we find are often distant, and identification is a challenge not because the species are similar but because they are too far away to see details. In these situations, any clue that helps to sort out the possibilities can be helpful. We all naturally look for color, and even very general impressions of light and dark are valuable. (For example, cormorants almost always look blackish.) Habits like diving are important clues, and overall shape — neck length, prominence of wingtips or tail, etc. — is also useful.
Another clue that is generally overlooked is flocking behavior. In the same way that flocking behavior varies in songbirds, it also differs consistently between species of waterbirds. Some species are often in large and dense flocks, and others only occasionally gather in small loose groups. Geese arrange themselves in different patterns than ducks, and Mallard flocks differ from scaup flocks, for example.