Apart from bats, birds are the only vertebrates capable of flight. The ancestors of modern birds took to the air some 150 million years ago, and since that time the ability to fly has allowed them to occupy almost every terrestrial habitat on Earth, and many aquatic ones too.
For birds, flight would not be possible without feathers, but these lightweight, tough and resilient structures are also vital for thermal insulation. In addition, contours, shapes, patterns and colors confer species and gender identity on their owners, and camouflage is also important for many species. Unsurprisingly, there are different feathers on a bird’s body that fulfill a range of functions, those associated with flight being structurally different from those that insulate.
In common with their reptilian ancestors, birds lay eggs, inside which their young develop. Eggs are laid in a nest; in the case of songbirds, the location and structure of the nest can range from a rudimentary cup-shaped structure sited in a cavity on the ground or in a treehole, to an intricately woven basket suspended from foliage. The eggs themselves are protected by a hard, chalky outer shell and the developing chick gains its nutrition from the egg’s yolk.