The Anatidae is the biological family that includes the ducks and most duck-like waterfowl (geese and swans). These are birds that are modified to swim, floating in the water surface, and dive in at least shallow water.
They have webbed feet and bills which are flattened to a greater or lesser extent. Their feathers are excellent at shedding water due to special oils. Duck, eider and goose feathers and down have long been popular for bedspreads, pillows, sleeping bags and coats. The members of this family also have long been used for food.
While the status of the Anatidae as a family is straightforward, and there is little debate about which species properly belong to it, the relationships of the different tribes and subfamilies are poorly understood. The listing in the box at right should be regarded simply one of several possible ways of organising the many species within the Anatidae.
The Anatidae can be seen as having six subfamilies:
- Dendrocygninae: whistling ducks. There are 9 species in the genus Dendrocygna which has a worldwide distribution through the tropics and subtropics, and a single diving species, Thalassornis leuconotus the White-backed Duck of Africa, which is difficult to classify and has often been placed in the Oxyurinae (which it resembles in morphology and feeding habits). It appears that the similarity is largely a matter of convergence, however.
- Oxyurinae: stiff-tail ducks and Freckled Duck. There are 6 species in the genus Oxyura (the typical stiff-tailed ducks) and three monospecific genera: Biziura (the Australian Musk Duck), Heteronetta (the South American Black-headed Duck which is well known because, Cuckoo-like, it is an obligate brood parasite) and Stictonetta (the Freckled Duck of southern Australia).
- Anserinae: geese and swans. There are 15 species of goose now often assigned to three genera (though the American Ornithologist's Union still separates at least one genus into two: Anser and Chen): Anser (9 species of 'grey' geese), Branta (5 species of 'black' geese, and the monotypic Cape Barren Goose. There are 24 species of swan in three genera: Cygnus, Coscoroba (South America), and Olor, the tundra swans.
- Tadorninae: shelducks, sheldgeese and steamer-ducks. This group of larger, often semi-terrestrial waterfowl can be seen as intermediate between geese (Anserinae) and ducks (Anatinae). There are 19 surviving species, mostly from the Southern Hemisphere, divided into 6 genera: Cyanochen Blue-winged Goose (Ethiopia), Chloephaga sheldgeese (South America, 5 species), Neochen Orinoco Goose (South America), Alopochen Egyptian Goose (Africa), Tadorna (the shelducks, 7 species), Lophonetta Crested Duck, (South America) and Tachyeres (the steamer ducks of South America).
- Anatinae: perching, diving, and dabbling ducks. The Anatinae are further divided into three groups:
- There are 19 species of perching duck in 15 genera (though the similarities in some are now regarded as convergent, which suggests that a reassignment may be appropriate).
- About 40 species of dabbling duck in 2 genera: the monotypic Marmaronetta (Marbled Duck) may instead be numbered among the pochards. The remaining 40-odd species are all part of the genus Anas, which includes wigeons, gadwalls, the true teals, pintails, mallards, and shovelers.
- The 16 species of pochards or diving ducks in 3 genera: Netta (3 species), Aythya (12 species), and Rhodonessa (the recently extinct Pink-headed Duck of India).
- Merginae: eiders, scoters, sawbills and other sea-ducks. All but two of the 20 species in this group occupy habitats in far northern lattitudes. There are 10 genera: Polysticta (Steller's Eider), Somateria (eiders, 3 species), Histrionicus Harlequin Duck, Camptorhynchus (the recently extinct Labrador Duck, Melanitta (the scoters, 3 species), Clangula (the Long-tailed Duck), Bucephala Goldeneyes (3 species), Mergellus Smew, Lophodytes (the Hooded Merganser of North America), and Mergus (the mergansers, 5 species, one extinct).