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