bird, warm-blooded, egg-laying, vertebrate animal having its body covered with feathers and its forelimbs modified into wings, which are used by most birds for flight. Birds compose the class Aves (see Chordata). There are an estimated 9,000 living species.
Birds are believed to be extant members of a group of dinosaurs called maniraptors (other maniraptors include Velociraptor and Oviraptor). They share with dinosaurs such characteristics as a foot with three primary toes and one accessory toe held high in back. Early birdlike animals include Arachaeopteryx, the ichthyornithiforms, skillful flyers with toothed beaks, Archaeornithura, and the rooster-sized, flightless Patagopteryx. The fossil remains of the Archaeopteryx, which date to the Jurassic period, show reptilian tails, jaws with teeth, and clawed wings, but feathers (found also in some dinosaurs) were well developed. Pterosaurs, another group of flying reptiles, did not share the common characteristics of birds and dinosaurs and are not considered birds. Whether the capacity for flight arose in tree-living dinosaurs that glided from branch to branch (the "trees-down" hypothesis) or in fast-running terrestrial dinosaurs (the "ground-up" hypothesis) continues to be debated. Indeed, the inclusion of birds in the dinosaur family tree, although accepted by most paleontologists, is debated by some, and the identification (2000) of feathers on a 220-million-year-old, four-legged reptile fossil, Longisquama insignis, raised questions concerning the theory. Archaeornithura, which dates to the Cretaceous period, is one of the earliest known ancestors of modern birds and resembles a modern wading bird.
Birds are of enormous value to humanity because of their destruction of insect pests and weed seeds. Many are useful as scavengers. The game birds hunted for food and sport include grouse, pheasant, quail, duck, and plover. The chief domestic birds are the chicken (see poultry), duck, goose, turkey, and guinea fowl. Parrots and many members of the finch family are kept as pets.
Characteristic Features and Behaviors
Like mammals, they have a four-chambered heart, and there is a complete separation of oxygenated and deoxygenated blood. The body temperature is from 2° to 14° higher than that of mammals. Birds have a relatively large brain, keen sight, and acute hearing, but little sense of smell. Birds are highly adapted for flight. Their structure combines lightness and strength. Body weight is reduced by the presence of a horny bill instead of heavy jaws and teeth and by the air sacs in the hollow bones as well as in other parts of the body. Compactness and firmness are achieved by the fusion of bones in the pelvic region and in other parts of the skeleton. The heavier parts of the body—the gizzard, intestines, flight muscles, and thigh muscles—are all strategically located for maintaining balance in flight. Feathers, despite their lightness, are highly protective against cold and wet. The flight feathers, especially, have great strength. Feathers are renewed in the process of molting. Some birds, such as the ostrich, the penguin, and the kiwi, lack the power of flight and have a flat sternum, or breastbone, without the prominent keel to which the well-developed flight muscles of other birds are attached. The bills of birds are well adapted to their food habits. Specialized bills are found in the crossbill, hummingbird, spoonbill, pelican, and woodpecker.
In the majority of species there are differences between male and female in plumage coloring. In these birds the male (except in the phalarope) is usually the more brilliant or the more distinctly marked and is the aggressor in courtship. Unusual courtship displays are performed by several species, particularly by the ruffed grouse, the bird of paradise, the crane, the pheasant, and the peacock. Birdsong reaches its highest development during the breeding season, and singing ability is usually either restricted to or superior in the male. Most birds build a nest in which to lay their eggs. Some birds, such as the oriole, weave an intricate structure, while others lay their eggs directly on the ground or among a few seemingly carelessly assembled twigs. Eggs vary in size, number, color, and shape. In spring and fall many birds migrate. Not all of the factors motivating this behavior are fully understood. These trips often involve flights of hundreds and even thousands of miles over mountains and oceans (see also migration of animals).
Among the periodicals devoted to the study of bird life are the Auk, the Condor, and the Wilson Bulletin. Books on birds include the many guides by R. T. Peterson; the life histories of North American birds in F. Gill and A. Poole, ed., The Birds of North America (1992–2003); R. M. De Schauensee, A Guide to the Birds of South America (1970); A. Rutgers and K. A. Norris, ed., Encyclopaedia of Aviculture (3 vol., 1970–77); U.S. Bureau of Sports Fisheries and Wildlife, Birds in Our Lives (1970); J. Van Tyne and A. J. Berger, Fundamentals of Ornithology (1971); S. Cramp, ed., Handbook of the Birds of Europe, the Middle East, and North Africa (5 vol., 1977–88); M. Walters, Birds of the World (1980); J. Farrand, Jr., Eastern Birds (1988) and Western Birds (1988); B. King et al., The Collins Field Guide to the Birds of South-East Asia (1988); S. Chatterjee, The Rise of Birds (1997); D. Attenborough, The Life of Birds (1998); P. Shipman, Taking Wing (1998); D. A. Sibley, The Sibley Guide to Birds (2000); T. Birkhead, The Wisdom of Birds: An Illustrated History of Ornithology (2009) and Bird Sense: What It's Like to Be a Bird (2012). A study of endangered birds and their habitats is Bird Watch (2011) by M. Walters.