ZOOLOGY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site bird Any member of a class of vertebrates distinguished by having feathers, two legs, and two wings. Birds are warm-blooded animals, and their young hatch from eggs. According to one theory, modern birds are descended from the dinosaurs. Many of the bones in a bird's skeleton are hollow to reduce the weight of the animal for flying. Like humans, a bird has ribs. These form a protective cage around the heart and lungs. The backbone and shoulder and hip girdles are adapted so that the weight of the body is carried on the wings in flying and on the legs in walking. The large flight muscles may account for one-fifth of the total body weight and are attached to a large, keeled breastbone and the upper part of the wing. A bird has large air sacs in the spaces in its body and even in the hollow bones. They are filled with the air that passes through the lungs when the bird breathes and are emptied through the lungs when the bird breathes out. In land animals, air flow is bidirectional: air travels through a network of channels, and stops at the alveoli where oxygen-carbon dioxide exchange takes place. Used air, containing carbon dioxide as a waste product, follows a reverse course in leaving the lung and is discharged through the windpipe. In birds, however, air flow is unidirectional. New air comes in one end, and the used air goes out the other end. This provides an uninterrupted supply of oxygen, necessary to meet a bird's high level of energy consumption. Birds have no teeth but a tough, horny beak. The food is not chewed but is stored in the crop after it is swallowed and is later ground up in the muscular gizzard which may contain stones. Related category    • ZOOLOGY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP