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.
　　鳥不在天空的高處飛。因為空氣太薄。
　　鳥很難呼吸稀薄的空氣。稀薄的空氣讓它們飛的并不持久。
　　鳥飛在地面附近,這樣它們可以看到它們在哪里。鳥兒知道它們要尋找的地方，所以它們不會迷路。有些鳥飛得很低以致于海洋波浪經(jīng)常淹沒了他們。許多鳥類在春天和秋天可以飛行很長的距離。