Description: Greater and lesser scaup are often found together, but the larger size of the greater scaup is very obvious. Male greater scaup also have a larger, more round, green-tinted head than male lesser scaup. Male greater scaup have a glossy black head tinted green. The neck, breast and upper mantle are glossy black, and the flanks and belly are white, sometimes with gray vermiculations on the lower flanks. The back is whitish with fine black vermiculations, and the tail and upper- and under-tail coverts are black. The wing has a broad white speculum spanning nearly the entire length of the primaries and secondaries. The bill is a light blue-gray with a black nail, the legs and feet are gray and the iris is yellow. Relatively silent except in display, the male greater scaup utters a soft cooing and whistling notes in courtship. Female greater scaup are brown with white oval patches around their bills. The female's bill is similar to that of the male, but slightly duller, and the legs and feet are gray. The female has harsh, gruff notes typical of the genus.

Breeding: Greater scaup breed on the tundra and in the boreal forest zones from Iceland across northern Scandinavia, northern Russia, northern Siberia and the western North American Arctic. It is estimated that three-quarters of the North American population breeds in Alaska. Greater scaup nest predominantly on islands in large lakes and lay an average of 9 eggs.

Migrating and Wintering: Greater scaup make extensive flights across the boreal forests of Canada prior to reaching their wintering grounds along the Atlantic coast and the Great Lakes, or migrate offshore from Alaska to their wintering grounds along the Pacific coast. Greater scaup occasionally are observed during winter in Central America and the Caribbean.

Population: Greater and lesser scaup are counted together, because they are difficult to distinguish during aerial surveys. Greater scaup are estimated to constitute roughly 11 percent of the continental scaup population. Scaup populations have steadily declined since the 1980s. Contaminants, lower female survival and reduced recruitment due to changes in breeding habitat or food resources are thought to be the primary factors contributing to the decline. The 2009 breeding population survey places the scaup population at about 4.2 million birds, an approximately 12-percent increase from the 2008 estimate (U.S. Fish and Wildlife Service, 2009).

Food habits: Greater scaup dive to feed on aquatic plants and animals. In coastal areas, mollusks constitute the principle diet items. In freshwater habitats, seeds, leaves, stems, roots and tubers of aquatic plants (sedges, pondweeds, muskgrass, wild celery, etc.) are important items.