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The sea ducks are a group of mostly arctic-adapted diving ducks that usually winter in coastal waters and typically breed in tundra situations or in northern forests. All twenty species (two of which are now extinct) depend predominantly on animal sources of food, and some feed exclusively on such materials. These foods include shellfish, mollusks, other invertebrates, and aquatic vertebrates such as fish. In general the sea ducks are thus not regarded as highly as table birds as are the surface-feeding ducks and some of the more vegetarian pochard species. Like the pochards, their legs are placed well to the rear and their feet are unusually large; thus the birds have sacrificed the ability to walk easily for their diving adaptations. Also in common with pochards, their generally heavier bodies relative to wing surface area prevent them from taking flight without running some distance over the water prior to reaching minimum flight speed. In the air they often make up in speed for their limited maneuverability, although some of the largest sea ducks are rather ponderous in flight. Some species exhibit a good deal of white on the wings while in flight, and, unlike the pochards, two species have iridescent speculum patterns. The arctic-breeding and tundra-nesting forms typically build open-cup nests in low vegetation, while the forest-nesting species often use hollow trees or other natural cavities for their nest sites. Some of these tree-nesting species have moderately long tails and can perch fairly well, but the larger eiders and scoters rarely stray far from the water's edge and are rather helpless on land.
Of the total of twenty species of sea ducks, North America is well endowed with fifteen extant breeding species, as well as the extinct Labrador duck. Further, the Old World smew has been reported several times in recent years, so that the only species not reported from North America are two Southern Hemisphere mergansers and an Asian species of merganser. Most of the North American species also occur extensively in the Old World, with the bufflehead, surf scoter, Barrow goldeneye, and hooded merganser being the exceptions.