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.