Birds are terrestrial vertebrates with feathers.
Feathers are the distinctive feature of birds: all birds have them and no other
animals are so covered. The forelimb is modified into wings for flight, leaving
the hindlimbs for walking (bipedal locomotion). Birds are built for flight;
special adaptations in body structure effect lightness in weight, efficiency,
and strength. Not only are the feathers light in weight and easily moved and
lifted by wind, but they also create warmth next to the body. Body heat warms
the air that is in contact with the bird's body. Warm air becomes lighter and
rises. Other adaptations for flight are the compact, but hollow, bones, numerous
air sacs occupying all available body spaces, reduced rectum, loss of teeth, and
feathers replacing a bony tail.
