Have you ever wondered why birds sing? Maybe you thought tha

Have you ever wondered why birds sing? Maybe you thought that they were just happy. After all，you probably sing or whistle when you are happy.
Some scientists believe that birds do sing some of the time just because they are happy. However，they sing most of the time for a very different reason. Their singing is actually a warning to other birds to stay out of their territory.
Do you know what a ’territory’ is? A territory is an area that an animal，usually the male，claims(声称)as its own. Only he and his family are welcome there. No other families of the same kind are welcome. Your garden and house are your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten(威胁)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. This screaming is what we call a bird’s song，and it is usually enough to keep an outsider away.
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 of their own. Most of it has to do with attracting mates and setting up territories.
小题1: Some scientists believe that most of the time bird’s singing is actually _______.

A．away of warning	B．an expression of happiness
C．an expression of anger	D．away of greeting

小题2:What is a bird’s "territory"?.

A．A place where families of other kinds are not accepted.

B．A place where a bird may shut at the top of its voice

C．An area for which birds fight against each other.

D．An area which a bird considers to be its own.

小题3: Why do birds keep on singing at nesting season?

A．Because they want to invite more friends.

B．Because they want to find outsiders around.

C．Because their singing helps frighten outsiders away.

D．Because their singing helps remove their fears.

小题4:How does the writer explain bird’s singing?

A．By comparing birds with human beings.	B．By reporting experiment results.
C．By describing birds’ daily life.	D．By telling a bird’s story.