David Sibley has put up a good post on identifying scoters by their silhouette. Why is this a good thing to learn? In David’s own words-
The three North American scoters can be distinguished readily by details of the head and bill, but like many birds of open water they are often seen at a great distance in bad light, when details such as bill shape and plumage patterns cannot be ascertained. More subtle (but equally diagnostic) differences in silhouette then become important.
I’d recommend heading to his site and seeing if you can tell the three species apart. I find the head shape and bill shape of the three scoters very distinctive, which is excellent for identification because these features are more consistent across age/sex classes than plumage. Check out some of Alex’s photos below to compare the head and bill shapes of Black, Surf and White-winged Scoters.
Sibley Guides « Identifying Scoters by Shape