They possess distinct reptilian characteristics such as the amniotic egg and scales on the legs. Each part of the bird's anatomy is modified in some way that enhances flight. The bones have a honeycombed internal structure that provides strength while reducing weight. The beak is made of keratin and evolution has produced many shapes in relation to the bird's diet. Flying requires much energy production from an active metabolism.
Birds are endothermic with insulation provided by feathers and a fat layer.
Birds have efficient circulatory system with a four-chambered heart that segregates oxygenated blood from unoxygenated blood. They have efficient lungs with tubes connecting to elastic air sacs that help dissipate heat and reduce the body density. Birds also have a very well developed nervous system. Acute vision and well-developed visual and coordinating areas of the brain aid in flying. They show complex behavior especially during breeding season when elaborate courtship rituals are performed.