ANSERIFORM

any member of the bird order Anseriformes, which contains the waterfowl, or wildfowl, family Anatidae (ducks, geese, and swans) and the screamers (family Anhimidae). Waterfowl are typically stocky birds with medium to long necks. The bill is generally of medium length, somewhat flattened, and often has a hooked nail on the tip. There is no characteristic wing shape among the waterfowl, but all of these birds are capable of flight, and many undertake extensive migrations. The screamers are long-legged, and their very large toes are webbed only at the base. Their bill is more chickenlike than ducklike and has a pronounced downward hook. The anseriforms, along with the ratites, curassows, and tinamous, possess an intromittent male organ. While this is not homologous to the mammalian penis, it serves the same reproductive function. Its presence, and the concomitant ability to mate while swimming, undoubtedly helped the anseriforms as they diversified into their aquatic niches. Several species of waterfowl have been domesticated. The mallard (Anas platyrhynchos) has been domesticated for at least 2,000 years. The muscovy duck (Cairina moschata) was domesticated in pre-Columbian South America. The greylag goose (Anser anser) and swan goose (A. cygnoides) are the basis of the several varieties of domestic geese. Wild waterfowl were important to early man as a source of meat, feathers, fat, and down. In some parts of the modern world, sport hunting of waterfowl is a major leisure activity. There are three main feeding styles within the waterfowl: dabbling, diving, and grazing. Dabbling ducks feed on or just below the surface and are known for upending to reach food (mostly vegetation) on the bottom of shallow waters. Among the diving waterfowl the freshwater forms generally inhabit fairly shallow waters and feed mostly on the seeds, leaves, and roots of aquatic plants. The marine ducks inhabit deeper sea waters and live on fish and invertebrates. Waterfowl are typically social birds and have a well-developed set of formal displays and group cohesion signals; for instance, in many species a head-shaking or chin-lifting display is used to coordinate group takeoff. Swans and geese use vocalizations to maintain contact while in flight. Preening and maintenance of plumage is a major individual activity. Most waterfowl have highly developed, innate pair-forming and bonding displays which differ from species to species. Among the ducks, which generally take a new mate each season, various bows and poses such as the head-up-tail-up display exhibit plumage markings that are otherwise not usually seen. In species that mate for life, such as the swans and geese, displays serve to reinforce the pair bond. The triumph ceremony of geese, for example, in which both birds call and wave their heads, is commonly performed after the male has driven off a rival. Almost all waterfowl breed in water, which provides safety and isolation. Territoriality plays a part in mating. Nest site selection and nest building are usually done by the female who pulls together any vegetation within reach. The eggs are usually laid one per day, in the morning. The shells may be white, green, or brown and clutches of the various species average from 3 to 12 eggs. The incubation period ranges from 22 to 39 days and incubation is almost always the job of the female. Shortly after hatching the young waterfowl imprint on their mother. Imprinting is an instinctive response to attach to and follow the nearest large moving object. The down-covered waterfowl follow the mother from the nest within 24 to 48 hours and are soon swimming and feeding. Young ducks may fledge as early as five weeks and are then on their own. Larger species, however, may not fledge for up to five months and geese and swans complete their first migration in the company of the parents. All of these birds are fairly long-lived; in captivity ducks may live 20 years, geese and swans for more than 30. Migration is a characteristic of many anseriform species, increasing the ecological flexibility of waterfowl. However, as their habitats are being destroyed, many species are facing extreme population pressures. any member of the order Anseriformes, which includes the well-known ducks, geese, and swans (family Anatidae) and the little-known screamers (family Anhimidae). The Anatidae comprise about 142 species of medium to large birds, usually associated with aquatic or marine habitats and known collectively as waterfowl (in the United States) or wildfowl (in Europe). These birds are of interest to many people for varied reasons: to the romantic and the artist for their swift, massed flights and association with wild places; to the sport hunter as a difficult and worthwhile quarry; to the economist for exploitation as producers of meat and feathers; to the farmer as possible competitors for grass or as damagers of crops; to the medical worker as a possible reservoir of arthropod-borne viruses. The conservationist views them as biological indicators of the richness of wetland habitats and, like those habitats, as objects for urgent attention. To the zoologist they are a close-knit group in evolution, but one that has adapted into a wide variety of ecological niches. Additional reading Kurt M. Bauer and Urs N. Glutz von Blotzheim, Handbuch der Vgel Mitteleuropas, vol. 2 and 3, Anseriformes (196869), provides much wider geographic coverage than the name implies and is a complete sourcebook on the biology of Holarctic waterfowl. Classic and still useful works include J.G. Millais, The Natural History of the British Surface-feeding Ducks (1902), and British Diving Ducks, 2 vol. (1913); and John C. Phillips, A Natural History of the Ducks, 4 vol. (192226, reprinted 1986), covering many aspects of waterfowl biology. H. Albert Hochbaum, Travels and Traditions of Waterfowl (1955, reissued 1967), reviews waterfowl migration and orientation. Paul A. Johnsgard, Handbook of Waterfowl Behavior (1965), inventories behaviour patterns, especially social and sexual, and his Waterfowl: Their Biology and Natural History (1968), contains photographs, many coloured, of nearly every species. Joseph P. Linduska (ed.), Waterfowl Tomorrow (1964), contains authoritative articles on all aspects of waterfowl conservation, management, and research. Worldwide coverage is provided in Jean Delacour, The Waterfowl of the World, 4 vol. (195464, reissued 1973), a treatment of all species of Anatidae; Peter Scott et al., The Swans (1972), with coverage of all 8 species; Johnsgard's Ducks, Geese, and Swans of the World (1978); Frank S. Todd, Waterfowl (1979); Peter Scott, A Coloured Key to the Wildfowl of the World, rev. ed. (1988), containing coloured plates of all species of the Anatidae, with details of distribution and aids to identification; and Steve Madge and Hilary Burn, Wildfowl (1988). Regional studies include G.L. Atkinson-Willes and D.G. Salmon, Wildfowl in Great Britain, 2nd ed. (1986), on Anatidae distribution and populations; H.J. Frith, Waterfowl in Australia (1967), devoted to the waterfowl of the Southern Hemisphere; Hochbaum's The Canvasback on a Prairie Marsh, 2nd ed. (1959, reissued 1981), a classic species monograph; Frank C. Bellrose et al., Ducks, Geese & Swans of North America, 3rd ed. (1980), a thorough treatment, with excellent illustrations; and John Gooders and Trevor Boyer, Ducks of North America and the Northern Hemisphere (1986). Geoffrey Vernon Townsend Matthews The Editors of the Encyclopdia Britannica