*rds don't fly high up in the sky. the air is too thin.
it is hard for *rds to breathe in thin air. thin air doesn't hold them up.
*rds fly near the ground so that they can see where they are. the *rds look for places they know. then they do not get lost. some *rds fly so low over the ocean that the waves often hide them. many *rds 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。
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