Though they are also classified as reptiles, birds belong to their own class, Aves. Birds have many structural adaptations for flight. For instance, the body is streamlined to minimize air resistance, and the endoskeleton bones are light and hollow. Many of the bones are also fused to provide compact strength. To enable flight, birds have feathers, which are lightweight adaptations of reptilian scales. Feathers also insulate against the loss of body heat and water.

Birds are homeothermic, meaning they are able to maintain a constant body temperature. The rapid pumping of their four-chambered heart and a high blood-flow rate contribute to this characteristic. Insulating feathers also help maintain a constant body temperature.

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