Birds now live almost everywhere on Earth. There are over 8,000 species of birds. Birds have adapted their vertebrate skeleton for flight. Their bones and skull are very thin, making their bodies extremely light. To support flight also required other changes to their skeleton.
Obvious changes are the addition of wings.
Other changes are less obvious. 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 air is thinner.