why do brids fly south in the winter?

why do brids fly south in the winter? why do birds south in the winter? Why do birds fly south for winter what in the winter do not fly south? 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 In winter ( ) do not fly south riddle:why do birds fly south for winter why do swan fly south? In winter ,some birds fly to the south 为什么要用fly啊. Swallows fly to the south before winter ( ) 脑筋急转弯 Why do birds fly south? 脑筋急转弯：1.Why do birds fly to south?2.There is four mistake in