any duck that obtains its food by diving to the bottom in deep water rather than by dabbling in shallows (see dabbling duck). On the basis of kinship and to the degree that it likes a marine environment, a diving duck may be popularly called either a bay duck or a sea duck.

Bay ducks of the tribe Aythyini, family Anatidae (q.v.; order Anseriformes), include canvasback, redhead, scaup, and allied species (see pochard). They are found more frequently in estuaries and tidal lagoons than on the open sea.

Sea ducks are any of the approximately 20 species of the tribes Mergini and Somateriini. The term is fairly apt, although some birds frequent inland waters as well as seacoasts. Ducks of the tribe Mergini include the bufflehead (Bucephala, or Glaucionetta, albeola), closely related to the goldeneye (B., or G., clangula); the mergansers; the oldsquaw (Clangula hyemalis); and scoters (Melanitta, or Oidemia, species). The tribe Somateriini includes the eiders. See also eider; merganser.