The male Greater Scaup has white flanks, a black rump and breast, and barred gray back. He has a green-black iridescent head and light gray-blue bill. The adult in its second year has a yellow eye. The male in non-breeding plumage has a black head and breast, brown body, and black rump. The female is brownish overall, also with a yellow eye after the age of two. The female also has a white semi-circle at the base of the beak. The Greater and Lesser Scaup can be difficult to distinguish in the field. The Greater Scaup averages about 10% longer and 20% heavier than the Lesser Scaup. The Lesser Scaup has a peaked, angular head that the Greater Scaup lacks. The Greater Scaup has a larger bill with a more pronounced nail (tip of the bill) than the Lesser Scaup. Seen in flight, the white on the wings of the Greater Scaup extends into the primaries, where it is gray on the Lesser Scaup. Habitat, range, and season may help differentiate between the two species, as well as using a field guide and working with experienced observers.