Because in the winter it's much colder in north than that in south, the birds are not used to the cold and they have to fly to the south.冬天北方比南方寒冷得多,鸟儿们不习惯寒冷的天气,所以不得不飞到南...是脑筋急转弯Because they fly much faster and fell less exhausted than they walk走过去太累太慢。。