General anatomy of a bird

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 Archaeopteryx, 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. 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.

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