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.
Birds are dioecious with sexual reproduction and internal fertilization.
Eggs are laid and must be kept warm through brooding by the female, male or both depending on the species. Wings are airfoils, formed by the shape and arrangement of the feathers, that illustrate the same aerodynamic principles as airplane wings. Feathers are made of keratin and are extremely light and strong. Feathers evolved from the scales of reptiles and may have first functioned as insulation. Feathers also function to control air movements around the wing.