This moderately large diving duck is the only circumpolar Aythya, and one of few circumpolar duck species. Less is known about its biology than other North American Aythya— in part because of its relatively isolated breeding grounds but also because of difficulty distinguishing it from its close relative, the Lesser Scaup (A. affinis). In North America, most Greater Scaup nest in coastal tundra of the Arctic and Subarctic, especially western Alaska. Most migrate southeastward across North America to winter on shallow embayments along the northeast Atlantic Coast, particularly in Long Island Sound. Smaller numbers winter farther south along the Atlantic Coast to northern Florida and on the Pacific Coast, mainly from Vancouver, British Columbia, south to San Francisco Bay. The species often occurs in large flocks during molt, migration, and on its wintering grounds. The Great Lakes provide an important rest stop for migrants as they funnel through to the Atlantic Coast.
Up to 80% of the population concentrates in winter in the urbanized northern portion of the Atlantic Flyway—where these ducks are subject to physical disturbances, habitat degradation, and various contaminants. Apparent long-term decline of Greater Scaup wintering in this core region has raised concerns about the welfare of the species and management strategies, as has evidence of dietary changes and elevated levels of organic contaminants and some heavy metals in winter habitats, foods, and the ducks themselves.
At a distance, Greater Scaup are often indistinguishable from Lesser Scaup, and their similarity is a major challenge to waterfowl managers, who require accurate population information for each species to set harvest limits and develop management policies. Unfortunately, because of identification difficulties, the 2 species are combined during aerial (and most ground) population surveys, so changes in populations of either species are obscured—especially those of Greater Scaup, whose numbers are overwhelmed by the more abundant Lesser Scaup.
Life history of North American Greater Scaup as of the mid-1970s was reviewed in Bellrose 1976a and Palmer 1976 . More recently, Allen et al. ( Allen et al. 1999 ) and Afton and Anderson ( Afton and Anderson 2001 ) summarized information on the population status of Greater and Lesser scaup from the 1950s through 1997, based on several sets of U. S. Fish and Wildlife Service (USFWS) and Canadian Wildlife Service data-collection programs related to waterfowl management, including temporal population estimates and harvest levels summarized by region. Several food-habit studies, primarily from birds taken during fall and winter hunting periods, have been reported: continental U.S. ( Kubichek 1933 , Cottam 1939 ), British Columbia ( Munro 1941c , Vermeer and Levings 1977 ), Great Lakes ( Jones and Drobney 1986 , Wormington and Leach 1992 , Leach 1993 , Hamilton and Ankney 1994 ), and Long Island Sound ( Cronan 1957 , Wahle 1990 , Wahle and Barclay 1993 , Cohen 1998a , Eccleston 1999 ). In his volume on comparative waterfowl behavior, Johnsgard ( Johnsgard 1965 ) described major aspects of social behavior.