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warm-blooded vertebrate of the class Aves, unique in having feathers, the one major characteristic that distinguishes them from all other animals. Birds have a four-chambered heart , forelimbs modified into wings, a calcareous-shelled egg, and keen vision, the major sense they rely on for information about the environment. Their sense of smell is not highly developed, and auditory range is limited. Most birds are diurnal in habit. There are approximately 8,700 living species, and more than 1,000 extinct species have been identified from fossil remains.The ability to fly has permitted an almost unlimited radiation of birds, so that they are now found virtually everywhere on earth.
There are considerable differences in flying ability among various birds. Penguins cannot fly but spend much of their time in the water swimming with their paddlelike wings; such birds as ostriches and kiwis have rudimentary wings and are permanently afoot. At the other extreme are the long-winged swiftsand frigate birds that move from their perches only to fly, never to walk. Most birds alternate some walking or swimming with their flying.
Birds depend to a great extent on innate behaviour, responding automatically to specific visual or auditory stimuli. Even much of their feeding and reproductive behaviour is stereotyped. Feather care is vital to keep the wings and tail in condition for flying and the rest of the feathers in place where they can act as insulation. Consequently preening, oiling, shaking, and stretching movements are well developed and regularly used.
Auditory signals, like visual ones, are almost universal among birds. The most familiar vocalization of birds is that usually referred to as “song.” It is a conspicuous sound that is used, especially early in the breeding season, to attract a mate, to warn off another bird of the same sex, or both. As such it is usually associated with establishing and maintaining territories. Individual variation in songs of many species is well known, and it is believed that some birds can recognize their mates and neighbours by this variation. Many other types of vocalizations are also known. Pairs or flocks may be kept together by series of soft location notes. Alarm notes alert other individuals to the presence of danger; in fact, the American robin (and probably many other species) uses one note when it sees a hawk overhead and another when it sees a predator on the ground. Begging calls are important in stimulating parents to feed their young . Other calls are associated with aggressive situations, courtship, and mating. Nonvocal sounds are not uncommon. Some snipe and hummingbirds have narrow tail feathers that produce loud sounds when the birds are in flight. The elaborate courtship displays of grouse include vocalizations as well as stamping of the feet and noises made with the wings. Bill clapping is a common part of courtship in storks.
Most birds build nests in which the eggs are laid. Nests vary widely: they may be a scrape in the sand, a deep burrow, a hole in a tree or rock, an open cup, or an elaborately woven hanging structure. The materials with which nests are made also vary widely. Some nests are lined with small stones, others are built of dirt or mud with or without plant material. Sticks, leaves, algae, rootlets, and other plant fibres are used alone or in combination. Some birds seek out animal materials such as feathers, horsehair, or snakeskin.
All birds incubate their eggs, except megapodes, which depend on the heat generated by decaying vegetation or other external sources, and brood parasites, which lay their eggs in the nests of other species.
Incubation takes from 11 to 80 days, depending at least in part on the size of the bird and the degree of development at hatching. Most songbirds and members of some other groups are hatched nearly naked and helpless (altricial) and are brooded until well able to regulate their body temperature. They are fed by the parents until after they are capable of flight. The young of numerous other birds, such as chickens, ducks, and shorebirds, are hatched with a heavy coat of down and are capable of foraging for themselves almost immediately (precocial)