Birds

Birds first appeared about 150 million years ago and now live almost everywhere on Earth. There are over 8,000 species of birds. Birds have three major differentiating characteristics: wings for flight, feathers, and a beak rather than teeth.

Birds have adapted their vertebrate skeletons for flight. Their bones and skulls are very thin, making their bodies extremely light. Supporting flight also required other changes to their skeletons. An obvious change is the addition of wings. Other changes are less obvious: for example, the claws and muscles of a bird's foot are designed to lock and hold onto a perch even while the bird is sleeping.

A bird's respiratory system is also adapted to make it easier to breathe at high elevations, where the air is thinner.