riddle:why do birds fly south for winter

riddle:why do birds fly south for winter Why do birds fly south for winter 脑筋急转弯 Why do birds fly south? why do birds south in the winter? 谁能用英语回答这个脑筋急转弯的问题呢why do birds fly south in the winter why do brids fly south in the winter? why do swan fly south? 脑筋急转弯：1.Why do birds fly to south?2.There is four mistake in 英语翻译1.Why do birds fly to south?2.There is four mistake in t In winter ( ) do not fly south In winter ,some birds fly to the south 为什么要用fly啊. what in the winter do not fly south?