The Greater Scaup (Aythya marila), left has a more rounded head with more of a peak toward the front and a more noticeable white patch on the cheek. This pair is a composite of 2 ducks photographed in Feb04 in the outer banks of North Carolina. On scaup profiles you can try this: Measure the diameter of the head from the base of the bill to the back of the head. Compare that distance to the body length at the water line. Lesser Scaup has a body length of about four head diameters while Greater Scaup has a body length of about three head diameters. Scaup are easier to identify when facing directly towards you. Greater Scaup has much wider cheeks and broader dark nail on the bill than Lesser. Greater Scaup are bigger than Lesser, and have a bigger looking head, with a larger bill. The head is usually much rounder in profile, without a noticeable bump, and often has a green gloss or sheen in good light. The end of the bill has a black knob called a "nail", and this is often larger in size on the Greater Scaup. Lastly, the flank of the Greater Scaup often appears a bit lighter, and the wiggly lines (vermiculations) on the both the back and sides appear finer than on the Lesser Scaup. Head shape is usually considered key, so a rounded head is a Greater until proven otherwise, and the "bump" on the rear of the head indicates a Lesser. If they are swimming towards you the Greater Scaup has fatter looking cheeks, kind of like a chipmunk.