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.
Before you start looking at flocks, it’s important to distinguish between foraging behavior and roosting behavior. As a general rule in waterbirds, foraging birds disperse more widely, and resting birds gather in tighter groups. For example, many species (loons, most grebes, Pelagic Cormorant, puffins) are solitary when they are foraging, but these same species form loose groups when resting on the water. Ducks are more scattered when foraging but packed tightly together, often in large flocks, when resting. And of course, groupings are also influenced by food and local conditions.
Given all of that variation, it might seem pointless to even try to describe differences in flocking behavior, but it really can be useful. This is not a stand-alone field mark. You will rarely be able to identify a species simply by its flocking behavior. But along with general color, size, and habitat, flock characteristics can allow you to reach an identification of some very distant birds. It is one of the clues that experienced birders use, even subconsciously, to quickly sort out distant birds. You can start to learn these differences just by noticing the behavior of the birds at your local patch, and soon you’ll be identifying distant waterbirds more easily.