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Until recent classifications by Jean Delacour and others, the pochard group was not taxonomically distinguished from the more marine-adapted sea ducks, here included in the following tribe Mergini. Nevertheless, the pochards are a readily definable group of mostly medium-sized ducks that differ from their close relatives, the surface-feeding ducks, in several respects. Their legs are situated somewhat farther back on the body, so that they are less adept at walking on land; their feet and associated webs are larger, increasing diving effectiveness (reflected by the increased length of the outer toes); and their bills are generally broad, heavy, and adapted for underwater foraging. Depending on the species, the predominant food may be of animal or vegetable origin. Internally, the males have tracheal tubes that are variably enlarged, and in contrast to the typically rounded and entirely bony structure of the tracheal bulla, this feature is angular and partially membranaceous. No iridescent speculum is present on the wings, but in many species the secondaries are conspicuously white or at least paler than the rest of the wing. The birds nest closely adjacent to water and sometimes even above the water surface, on reed mats or similar vegetation.
North America has five well-distributed species of pochards, one of which (the greater scaup) also extends to the Old World. Additionally, North American tufted duck records have become so numerous in recent years that the inclusion of that species has seemed necessary. One other Old World species, the common pochard (Aythya ferina), has rarely occurred in Alaska, with several Aleutian Islands records in recent years (Byrd et al., 1974).