There are several black and white ducks that show up in Colorado, including the Common Goldeneye, Barrow’s Goldeneye, Bufflehead, and Ring-necked Duck. But few are as difficult to tell apart as the Greater Scaup (Aythya marila) and the Lesser Scaup (Aythya affinis).
The males of both birds have iridescent black heads (showing either green or purple in the right light), white sides, black rumps, and black and white barred backs that look grey from a distance. Both also breed in the arctic and can be found in the winter in Colorado, though the Greater Scaup is much rarer here owing to the fact that it prefers coastal areas.
So, you have a black and white duck that you’ve narrowed down to being a scaup. How do you figure out which scaup you have? The classic way to tell between the two is to look at the head shape of the bird. Is there a bump on the top of the head behind the eye? You have a Lesser Scaup. Is the head rounded or there’s a small bump on the top of the head in front of the eye? You have a Greater Scaup. The Greater Scaup also has a more extensive black nail on the bill than the Lesser Scaup.
But what if you can’t tell whether there’s a bump or where the bump is? Another way to tell is by the amount of white on the extended flight feathers of the wing. The bright white on the wing of the Greater Scaup extends from the secondaries (inner flight feathers) out onto the primaries (outer flight feathers), while the bright white on the wing of the Lesser Scaup is restricted almost entirely to the secondaries. So if you can wait for the bird to fly or stretch, you’ll know immediately which scaup you’re seeing.
If you’re lucky to have both in the same flock, you can also easily see the size difference between the birds. Greater Scaups are slightly larger with proportionately bigger heads. They are also usually brighter white on the sides than Lesser Scaups, though this isn’t always the case.
Now, can you find the Lesser Scaup in the photo below?