Turtles, crocodiles and the external structure of birds

Birds are warm-blooded animals adapted to the air: the body is covered with feathers, the bones are light, the front limbs have become wings, and the jaws form a horny beak. Body temperature does not depend on the outside, the heart has four chambers, the lungs work together with air sacs, and birds reproduce by laying eggs. Bird structure is studied on the rock dove. The dove’s body consists of head, neck, trunk and legs. The round head is joined to the trunk by a long movable neck; the nostril is on the beak, and the eyes and ear openings are on the sides of the head. The spread wing forms a lifting surface; its skeleton has upper-arm, forearm and hand bones. The lower leg and toes are covered with rough skin with horny scales, and the bird walks on its legs only. Feathers are divided into contour feathers and down. Contour feathers are body-covering, flight (wing), steering (tail) and fluffy feathers. A feather has a thin, hard shaft and a vane on both sides; the vane is made of twice-branching barbs, neighbouring barbs are joined with tiny hooks, so the vane is light, flexible and airtight. In flight the flight and steering feathers overlap like roof tiles. Fluffy feathers are soft and porous and lack the second-order barbs; water birds also have down, which retains heat well. In moulting old feathers fall and new ones grow. Birds have few skin glands; most birds have a preen gland, especially well developed in water birds, that makes oil with which the bird greases its feathers to make them waterproof. Feathers, beak and claws are horny, and leg scales resemble reptile scales.