Anatidae (ducks, geese, mergansers, pochards, sawbills, swans; class Aves, order Anseriformes) A family of mainly aquatic birds that have flat, lamellate bills, except for the six species of Mergus, the mergansers or sawbills, which feed on fish and have long, narrow, serrated bills, and shelducks (seven species of Tadorna), which are fairly large birds that resemble geese, and have short, slightly upturned bills. The front toes are webbed (the more terrestrial and non-migratory Branta sandvicensis, the Hawaiian goose or ne-ne, has reduced webbing and short wings). Many Anatidae show sexual dimorphism. They have thick feathers with insulating down. (Stifftails (six species of Oxyura) have long, stiff tail feathers and the males have long, blue bills.) The flight feathers are moulted simultaneously after breeding. They feed on vegetable and animal foods and nest on the ground or in holes in trees, among rocks, or in the earth, and the nest is usually lined with down. Eiders (three species of Somateria) are sea ducks, found in estuaries and coastal areas, as are scoters (three species of Melanitta) although these breed inland. Whistling ducks (eight species of Dendrocygna) are partially nocturnal. The largest genus, with 36 species, is Anas (dabbling ducks); A. platyrhynchos (mallard) is the ancestor of most domestic ducks. There are nine or 10 species of Anser (geese); A. anser (greylag goose), A. cygnoides (swan goose), and Cygnus olor (mute swan) are also extensively domesticated. C. atratus (black swan) has been introduced to New Zealand. Branta canadensis (Canada goose) has been introduced into Europe. There are 12 species of Aythya (pochards), some of which feed in sea water. Anseranas semipalmatis (magpie goose) occurs on the floodplains of northern Australia. There are 43 species, with cosmopolitan distribution.