The Greater Scaup, left has a more rounded head with more of a peak toward the front and a more noticeable white patch on the cheek. These 2 were together in northern Minnesota Oct04. 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.