The bills of birds are well adapted to their food habits. 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. Feathers are renewed in the process of molting. The flight feathers, especially, have great strength. Feathers, despite their lightness, are highly protective against cold and wet. The heavier parts of the body-the gizzard, intestines, flight muscles, and thigh muscles-are all strategically located for maintaining balance in flight. Compactness and firmness are achieved by the fusion of bones in the pelvic region and in other parts of the skeleton. 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. Their structure combines lightness and strength. Birds have a relatively large brain, keen sight, and acute hearing, but little sense of smell. The body temperature is from 2° to 14° higher than that of mammals. Like mammals, they have a four-chambered heart, and there is a complete separation of oxygenated and deoxygenated blood. Parrots and many members of the finch family are kept as pets. The chief domestic birds are the chicken (see poultry), duck, goose, turkey, and guinea fowl. The game birds hunted for food and sport include grouse, pheasant, quail, duck, and plover. Archaeornithura, which dates to the Cretaceous period, is one of the earliest known ancestors of modern birds and resembles a modern wading bird.īirds are of enormous value to humanity because of their destruction of insect pests and weed seeds. Indeed, the inclusion of birds in the dinosaur family tree, although accepted by most paleontologists, is debated by some. 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.

Pterosaurs, another group of flying reptiles, did not share the common characteristics of birds and dinosaurs and are not considered birds. 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. Early birdlike animals include Archaeopteryx, the ichthyornithiforms, skillful flyers with toothed beaks, Archaeornithura, and the rooster-sized, flightless Patagopteryx. They share with dinosaurs such characteristics as a foot with three primary toes and one accessory toe held high in back. There are an estimated 9,000 living species.īirds are believed to be extant members of a group of dinosaurs called maniraptors (other maniraptors include Velociraptor and Oviraptor). Birds compose the class Aves (see Chordata). 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.

Wikipedia bird box