___1____ Maybe you thought that they were just happy. After all, you probably sing or whistle when you are happy.
However, they sing most of the time for a very different reason. ____2___ Some scientists believe that birds do sing some of the time just because they are happy.
Do you know what a “territory” is? A territory is an area that an animal, usually the male, claims(声称)as its own. ____3___ No other families of the same species are welcome. Your yard and house are your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten you, you might shout. Probably this would be enough to frighten him away. If so, you have actually scared the stranger away without having to fight him. A bird does the same thing. But he expects an outsider almost any time, especially at nesting(筑巢)season. So he is screaming all the time, whether he can see an outsider or not.__4__.
Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter the territory of theirs. You can see that birds have a language all their own. ___5__

A．Only he and his family are welcome there.

B．Most of it has to do with attracting mates and setting up territories.

C．Have you ever wondered why birds sing?

D．Their singing is actually a warning to other birds to stay out of their territory.

E. Have you ever wondered why birds sing?
F. Most of the language is widely used.
G. This screaming is what we call a bird’s song, and it is usually enough to keep an outsider away.