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.
鸟不在天空的高处飞。因为空气太薄。
鸟很难呼吸稀薄的空气。稀薄的空气让它们飞的并不持久。
鸟飞在地面附近,这样它们可以看到它们在哪里。鸟儿知道它们要寻找的地方，所以它们不会迷路。有些鸟飞得很低以致于海洋波浪经常淹没了他们。许多鸟类在春天和秋天可以飞行很长的距离。
