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Anatidae
The Anatidae are the biological family of birds that includes ducks, geese and swans. The family has a cosmopolitan distribution, occurring on all the world's continents. These are birds that are adapted through evolution for swimming, floating on the water surface, and in some cases diving in at least shallow water. (The magpie goose is no longer considered to be part of the Anatidae, but is placed in its own family Anseranatidae.) The family contains around 146 species in 40 genera. They are generally herbivorous, and are monogamous breeders. A number of species undertake annual migrations. A few species have been domesticated for agriculture, and many others are hunted for food and recreation. Five species have become extinct since 1600, and many more are threatened with extinction.
==Description and ecology==
The ducks, geese and swans are small- to large-sized birds with a broad and elongated general body plan.〔 Diving species vary from this in being rounder. Extant species range in size from the cotton pygmy goose, at as little as 26.5 cm (10.5 inches) and 164 grams (5.8 oz), to the trumpeter swan, at as much as 183 cm (6 ft) and 17.2 kg (38 lb). The wings are short and pointed, and supported by strong wing muscles that generate rapid beats in flight. They typically have long necks, although this varies in degree between species. The legs are short, strong, are set far to the back of the body (more so in the more aquatic species), and have a leathery feel with a scaly texture. Combined with their body shape this can make some species awkward on land, but they are stronger walkers than other marine and water birds such as grebes or petrels. They have webbed feet. The bills are made of soft keratin with a thin and sensitive layer of skin on top (which has a leathery feel when touched). For most species, the shape of the bill tends to be more flattened to a greater or lesser extent. These contain serrated lamellae which are particularly well defined in the filter-feeding species.〔
Their feathers are excellent at shedding water due to special oils. Many of the ducks display sexual dimorphism, with the males being more brightly coloured than the females (although the situation is reversed in species such as the paradise shelduck). The swans, geese and whistling-ducks lack sexually dimorphic plumage. Anatids are vocal birds, producing a range of quacks, honks, squeaks, and trumpeting sounds, depending on species; the female often has a deeper voice than the male.〔
Anatids are generally herbivorous as adults, feeding on various water-plants, although some species also eat fish, molluscs, or aquatic arthropods. One group, the mergansers, are primarily piscivorous, and have serrated bills to help them catch fish. In a number of species, the young include a high proportion of invertebrates in their diets, but become purely herbivorous as adults.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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