This hardy eider, the largest duck in the Northern Hemisphere, is more closely tied to marine habitats than any other sea duck. It generally inhabits arctic and subarctic coastal marine habitats and has a circumpolar distribution. Its tendency to nest in large colonies on marine islands and to form large aggregations in inshore coastal areas during the nonbreeding season makes this species truly marine in all aspects of its life history. Many populations remain as far north as open water persists in winter where they are associated with polynyas (openings in the ice) and leeward sides of islands free from moving pack ice. Common Eiders are diurnal feeders and dive to pick mollusks and crustaceans from the sea bottom in water depths generally ranging from 10 to 20 meters.
Females are faithful to their natal and breeding areas and frequently reuse the same nest site. Philopatry to wintering sites is likely but unstudied. Common Eiders are known to be seasonally monogamous, and in some populations show long-term pair bonds, while in other populations, they do not.
This species has been exploited throughout its range. Market hunting almost extirpated the southern, or American, race (S. m. dresseri) from the eastern seaboard of North America by the end of the nineteenth century. The Migratory Bird Convention (1916) designated special protection to eiders (Article IV), and largely terminated this excessive commercial hunting. This population is currently healthy but is under increasing harvest pressure. Populations of the Pacific (S. m. v-nigrum), northern (S. m. borealis), and Hudson Bay races (S. m. sedentaria) have declined.
The Common Eider is a well-studied sea duck, mainly because of its immense numbers and profile, its economic importance, and its significance in subsistence harvests in the north (for example, approximately 2,000 references cited in Milne and Dau 1976 ). Key studies are apparent for population dynamics ( Milne 1974 , Reed 1975a , Swennen 1976 , Coulson 1984 , Krementz et al. 1996 ), juvenile production and habitat ( Milne and Reed 1974 , Minot 1980 ), juvenile predation and survival ( Mendenhall and Milne 1985 , Swennen 1989 ), husbandry and conservation ( Doughty 1979 , Krohn et al. 1992 , Goudie et al. 1994a ), habitat selection ( Guillemette et al. 1993 ), contaminants ( Henny et al. 1995 ), physiology ( Korschgen 1977 , Jenssen et al. 1989 , Parker and Holms 1990 , Gabrielsen et al. 1991a ), parasites ( Bishop and Threlfall 1974 , Persson et al. 1974 ), behavior ( McKinney 1961 , Gorman and Milne 1972 , Ydenberg and Guillemette 1991 , Frimer 1995b ), crèching ( Munro and Bédard 1977a ), phylogeny ( Livezey 1995b ), subspecies ( Mendall 1986 ), and food habits ( Madsen 1954 , Pethon 1967 , Player 1971 , Goudie and Ankney 1986 , Nystrom et al. Nyström et al. 1991 ).