Two subspecies of scaup exist in the U.S., both predominantly black and white from bill to tail. They can be difficult to tell apart, particularly in flight, but there are differences. The larger of the two at 20 to 22 inches, the greater scaup is white from its shoulders to the tips of the primary and secondary feathers. The lesser, 16 to 18 inches, is white only to the midpoint of the wing. In the right light, the greater’s head glows with a green iridescence, whereas the lesser’s shines a dark purple. Both have heavy blue bills, but the greater’s black tip is broad, while the lesser’s is narrow and slightly hooked. Hens of both subspecies range from brown to grayish brown, with the same wide heavy bill and white facial patch at the base.