Birds don't give birth to live ones. They are warm-blooded and lay eggs. Their bodies are covered with feathers and they have wings. Feathers have three functions: flight, temperature regulation and display. Most birds have hollow main bones with air sacs in them. This makes them lighter and makes flight easier.Birds are bipedal: they have two legs which are often covered with scales (small, flat plates which over-lap in the same way as feathers). They have a hard beak with no teeth.