閱讀理解。

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 as its

own .Only he and his family are welcome there .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's screaming (高聲尖叫) 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 something to do

with attracting mates and setting up territories.

1.Some scientists believe that most of the time birds' singing is actually .

A.an expression of happiness

B.a way of warning

C.an expression of anger

D.a language of their own

2.What is a bird's "territory"?

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

B.A place where other bird families are not accepted.

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

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

3.Why do birds keep on singing at nesting season?

A.Because their singing helps frighten outsiders away.

B.Because their singing helps get rid of their fears.

C.Because they want to find outsiders around.

D.Because they want to invite more bird friends.

4.How does the writer explain birds' singing?

A.By comparing birds with human beings.

B.By reporting experimental results.

C.By describing birds'daily life.

D.By telling a bird's story.