The Greater Scaup (Aythya marila), just Scaup in Europe, or colloquially known as "Bluebill", is a small diving duck. It breeds on the ground by lakes and bogs on the tundra and at the northern limits of the boreal forest across Arctic and subarctic regions of northern North America, Europe and Asia.
The adult Greater Scaup is 42–51 cm long with a 71–80 cm wingspan, larger than the Lesser Scaup. It has a blue bill and yellow eyes. The male has a dark head with a green sheen, a black breast, a light back, a black tail and a white bottom. The adult female has a white band at the base of the bill and a brown head and body.
Nearctic Greater Scaup are separable from Palaearctic birds by stronger vermiculation on the mantle and scapulars, and are considered a separate subspecies, A. m. nearctica. Based on size differences, a Pleistocene paleosubspecies, Aythya marila asphaltica, has also been described from fossils recovered at Binagady, Azerbaijan.
Greater Scaup migrate southwards to winter in flocks to coastal waters.
The Greater Scaup mainly eats mollusks and aquatic plants, obtained by diving and swimming underwater. There is a report of four Greater Scaups swallowing leopard frogs (with body length about 5 cm (2 inches)) which they dredged out of a roadside freshwater pond.
The Greater Scaup's name may come from "scalp", a Scottish and Northern English word for a shellfish bed ("probably" the same word as the scalp of the head), or from the duck's display call scaup scaup. It is usually silent when not breeding.
The Greater Scaup is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies.
In North America, Greater Scaup populations have been on a steady decline since the 1990's. Biologists and conservationists are unsure of the reasons for decline. Some researchers believe a parasitic trematode found in snails may be to blame.