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When floating far out on the water, it can be difficult to separate the Greater Scaup (18 inches) from its relative, the Lesser Scaup (Aythya affinis) (16 1/2 inches). Males of both species are medium-sized ducks with dark heads and chests, light backs and flanks, and dark tails. A closer look reveals that the Greater Scaup has a flat-topped, green-tinged head and white flanks (as opposed to the Lesser Scaup, which has a peaked, purple-tinged head and light gray flanks). The females of both species (both dark brown) are also difficult to separate, although the female Greater Scaup tends to be slightly lighter brown than the female of the other species. Both species also have blue bills, earning them the nickname “bluebill” with duck hunters. The Greater Scaup is found across much of the Northern Hemisphere. In North America, the Greater Scaup primarily breeds in western Alaska, in northeastern Canada, and along the Hudson Bay. Greater Scaups migrate south in winter, when they may be found along the Pacific coast from Alaska to Baja California, on the Atlantic coast from Newfoundland to Florida, on the northern Gulf coast, and in the Great Lakes. The Greater Scaup is more of a coastal bird than the Lesser Scaup in winter, but may be found inland on migration. In the Old World, this species breeds in Iceland, Scandinavia, and Russia, wintering south along the coast to the Mediterranean Sea, the Persian Gulf, and coastal China. Greater Scaups breed on freshwater wetlands with marsh grasses. In winter, they may be found in large numbers in offshore waters and in large bays. Although they may also be found in saltwater in winter, Lesser Scaup are somewhat less likely to be seen on the open ocean than Greater Scaup. This species’ diet primarily consists of invertebrates, such as crustaceans, mollusks, and insects when available. One of several species of “diving ducks” in North America, Greater Scaups may be observed submerging themselves to feed on invertebrates in the water or on the bottom. In winter, they may also be observed in flocks of many hundreds or thousands of birds offshore. This species is primarily active during the day.