Very few of our birds stay with us the year round.Some come to us in the winter from the cold?north.Others come from the south to spend the summer with us.How do they know the way?Suppose?you were told to find your way to a place hundreds of miles away,do you think you could do it?
Yet birds travel over mountains,forests,lakes and even across the oceans,and do not stray from?the path.They find their way back in the spring to the same orchard（果园）and the very trees where?they nested the summer before.
It is wonderful how quickly birds travel such long distances from their summer homes to their?winter ones.Some birds have been known to fly hundreds of miles in a day.But others travel much?more slowly.
Why do birds undertake these long journeys twice a year?Perhaps cold weather and lack of?food drive them from us in the autumn,but we cannot tell why they leave the sunny south to come?back to us in the spring.We know only that many of them like to make their nests and rear their?young in the north.
We are sorry to see them go,but we know that when winter is over they will come back to us.
Why do birds undertake those long journeys twice a year?
A.It is because of cold weather in winter
B.It is because of lack of food in autumn
C.It is because of the hot weather in summer
D.There's some reason that is still not known clearly