Drake greater scaup are larger and have more rounded heads than the females; they have a bright blue bill and yellow eyes.

Greater scaup nest near water, typically on islands in northern lakes or on floating mats of vegetation.

The drakes have a complex courtship, which takes place on the return migration to the summer breeding grounds and concludes with the formation of monogamous pairs.

Greater scaup eat aquatic molluscs, plants, and insects, which they obtain by diving underwater to depths of 0.5–6 m, exceptionally 10 m.[4] They form large groups, called "rafts", that can number in the thousands.

[5][6] The species is now placed in the genus Aythya that was introduced for the greater scaup by the German zoologist Friedrich Boie in 1822.

The birds in North America are treated as a separate subspecies A. m. nearctica,[10] and are distinguishable from those in Europe by a typically higher forehead, and the male having stronger vermiculations on the mantle and scapulars.

[12] Based on size differences, a Pleistocene paleosubspecies, Aythya marila asphaltica, has also been described by Serebrovskij in 1941 from fossils recovered at Binagady, Azerbaijan.

It has a light blue bill with a small black nail on the tip, yellow eyes, and is 20% heavier and 10% longer than the closely related lesser scaup.

The adult female has a brown body and head, with white wing markings similar to those of the male but slightly duller.

It has a white band and brown oval shaped patches at the base of the bill, which is a slightly duller shade of blue than the drake's.

The North American subspecies A. m. nearctica typically has a higher forehead and reduced white on the wings, intermediate between the European A. m. marila and Lesser Scaup.

[16] The greater scaup has a circumpolar distribution, breeding within the Arctic Circle both in the Old World (the Palearctic) and in North America (the Nearctic).

Drake greater scaup have a soft, quick whistle they use to attract the attention of hens during courtship, which takes place from late winter to early spring, on the way back to their northern breeding grounds.

[25] Pairs nest in close proximity to each other in large colonies, usually near water, on an island or shoreline, or on a raft of floating vegetation.

These lakes can be close to the breeding grounds, or distant, with birds performing a moult migration of up to several hundred km, such as from northern Scandinavia to the IJsselmeer in the Netherlands.

[16] There is a report of four greater scaups in April near Chicago swallowing hibernating leopard frogs (a species with a body length about 5 centimetres, or 2.0 in), which they dredged out of a roadside freshwater pond.

[30] In freshwater ecosystems, the greater scaup will eat seeds, leaves, stems and roots, along with sedges, pondweeds, muskgrass, and American eelgrass.

[17] Owing to the greater scaup's webbed feet and weight, it can dive up to 6–10 metres (20–30 ft) and stay submerged for up to a minute.

[31] In the past, they also often fed in winter in huge flocks on spent grain pumped offshore from distilleries (notably in the Firth of Forth in Scotland), but with modern pollution control, this food source is no longer available.

[19] Although the greater scaup faces numerous threats, the most significant challenge to their survival is habitat degradation caused by a mix of human development and runoff.

On their migration across the Great Lakes, greater scaups are at risk of ingesting selenium by eating the invasive zebra mussels, which can render the female infertile.

[38] They are hunted in Denmark, Germany, Greece, France, the United Kingdom, and Ireland,[38] and in Iran for both sport and commercial reasons.