Birds don't fly high up in the sky. The air is too thin.

　　It  is  hard  for  birds  to  breathe  in  thin  air.  Thin air doesn't hold them up.

　　Birds fly near the ground so that they can see where they are. The birds look for places they know. Then they do not get lost. Some birds fly so low over the ocean that the waves often hide them. Many birds fly a long distance in the spring and autumn.