Aythya Marila male Greater Scaup has white flanks, a black rump and breast, and barred gray back. He has a green-black iridescent head and light gray-blue bill. The adult in its second year has a yellow eye. The male in non-breeding plumage has a black head and breast, brown body, and black rump. The female is brownish overall, also with a yellow eye after the age of two. The female also has a white semi-circle at the base of the beak. The Greater and Lesser Scaup can be difficult to distinguish in the field. The Greater Scaup averages about 10% longer and 20% heavier than the Lesser Scaup. The Lesser Scaup has a peaked, angular head that the Greater Scaup lacks. The Greater Scaup has a larger bill with a more pronounced nail (tip of the bill) than the Lesser Scaup. Seen in flight, the white on the wings of the Greater Scaup extends into the primaries, where it is gray on the Lesser Scaup. Habitat, range, and season may help differentiate between the two species, as well as using a field guide and working with experienced observers.
Outside of the breeding season, Greater Scaup form large flocks or rafts, numbering in the thousands. In tidal waters, they tend to face up-current. While individuals may drift downstream, birds from the back of the flock fly to the front, maintaining the raft in the same position. A diver, the Greater Scaup catches its food under water, but eats it on the surface. Occasionally scaups forage at or near the water’s surface as well.

Aythya marila breeds in northern Europe, and winters mainly in north-western Europe and the Black Sea, which together account for less than half of its global wintering range. Its European wintering population is large (120,000 individuals), and was stable between 1970-1990. Although most wintering populations were broadly stable during 1990-2000, there were substantial declines in north-western Europe, and the species probably underwent a very large decline (50%) overall.
This duck has a wide distribution at high latitudes in Eurasia and North America. In Europe its mainly marine wintering area is reaching the Mediterranean and the Black Sea, but most birds winter in the Baltic and North Seas. This north-western European population is estimated at 310000 individuals. A decline has been recorded in the British Isles and in Denmark; an increase in the Netherlands and Germany. Its global trends are unknown.

The Greater Scaup is the more northerly of the two species of North American Scaup. In the summer, they breed on marshy, lowland tundra at the northern limits of the boreal forest. In winter, they gather in coastal bays, lagoons, and estuaries, with some wintering on inland lakes. While the Greater Scaup does overlap with the Lesser Scaup in winter, it tends to frequent more open, exposed areas.

While Greater Scaup may nest at one year of age, they are more likely to begin breeding at the age of two. Pair formation begins in late winter or early spring on the wintering grounds and during migration back to the breeding grounds. The pair bond lasts until the female begins to incubate, and then the male leaves. Nests are located close to the water on an island, shoreline, or floating mat of vegetation, often close together in loose colonies. The female makes a shallow depression lined with grass. During incubation, the female adds down to the nest. Incubation of the 6 to 9 eggs lasts for 23 to 28 days. Soon after they hatch, the female leads the young to the water. Multiple broods may form small crèches and be tended by one or more females. The young can swim and find their own food immediately, but can’t fly until about 40 to 45 days old

Migratory, winters along coasts of North America (Atlantic and Pacific), NW Europe, Turkey, Black and Caspian Seas and Japan, Yellow and East China seas. At some localities (e.g. Central Europe, Great lakes of North America) also inland. Males to remain much further  than females or immatures.

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