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This hardy, ornately plumaged sea duck breeds in remote areas of both the New and the Old World Arctic, spending only a few weeks (males) to several months (successfully breeding females) on land. Hatch year birds spend their first year at sea. Moving south in fall ahead of or with advancing sea ice, many winter at the southern extent of the ice, but occasionally immatures venture as far south as Florida and southern California. Adults are among the first birds to return north to breeding areas in spring, arriving before ice and snow have completely melted. Few birds of any other species nest farther north than the King Eider.
There are 2 populations of King Eiders in North America—one that winters in the west, the other in the east. Their migrations are some of the most spectacular passages of birds on the continent. In coastal northwestern Alaska, flocks of tens of thousands may pass northward during spring migration, often interspersed with other waterfowl and seabirds; such passages can last uninterrupted for hours. Molt and fall migrations can also be impressive, but they are extended over summer and fall.
Similar to other eider species, King Eiders dive to forage on marine invertebrates while at sea. They also forage on freshwater invertebrates in tundra ponds during the breeding season. Also similar to other eiders, female King Eiders are faithful to their breeding areas. Recent evidence also indicates natal site fidelity for females.
Because of the remote habitats of the King Eider, information on this species was based on relatively few studies and anecdotal observations until the advent of advanced tracking devices in the late 1990s. Recent studies using satellite tracking to determine the location and timing of migration, molt, and wintering areas indicate that King Eiders have a wide range of movements during the non-breeding season, and individual migration strategies vary widely. Unlike Common (Somateria mollisima), Spectacled (S. fischeri), and Steller's Eiders (Polysticta stelleri), western King Eiders winter over a broad area in the Bering Sea, ranging from 50° to 65° N latitude.
Over the first decade of the 2000s, numerous studies on the breeding grounds in Canada and Alaska led to more information on nest and duckling survival, contributions of exogenous nutrients to eggs, incubation behavior, and contaminant loads. There are still few data on the proportion of non-breeders, age of first breeding, and lifetime reproductive success.
Although King Eiders are relatively plastic in their use of at-sea habitats, it is still unclear how changes in sea ice extent, and prey composition and availability will affect the species as climate changes in the Arctic. More research on forage quality and quantity is needed, especially for key winter, molt, and staging areas. Because of the extensive time spent at sea, and some indications of declining populations, potential impacts of offshore gas and oil exploration and extraction, and alternative energy such as wind farms, are also of conservation concern.