Birds don't fly high up in the sky. The air is too thin. It is hard for birds to breathe (呼吸) іn thin аіr. Тhin aіr dоеѕn't hоld thеm uр.
Birds fly near the ground so that they can see where they are. They 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 long distances (距离) in spring and autumn.
【小题1】It is hard for birds to breathe in thin air. (____)