A、Short and unchanging. B、Pleasant and melodious. C、Melodious and complicated. D、Pleasant and changing. A

The sound of birds can be divided into 3 types including singing, crying and voice imitating. Singing is normally a continuous multi-syllable melody uttered by the birds under the control of sex hormone. Some sorts of singing are quite melodious and pleasant. During the breeding cycle, the changing and pleasant cries uttered by male birds are typical singings. Singing is an important way for a bird to defend its territories, to pronounce it has occupied a place and to warn other birds not to enter the area. The songs uttered by birds in singing are more different and complicated than their common cries, which mainly happen in the breeding cycle in spring and summer. Cries are not controlled by sex hormone and can be uttered by both male and female birds. Usually cries of birds are short and simple. But the cries of birds can mean a lot. They are used to communicate with one another and to inform one another about dangerous information. The cries of birds can express calling, warning, surprising and threatening. Sometimes it is very difficult to distinguish crying and singing. Generally speaking, cries are not affected by the change of seasons and can be uttered by all birds; Crying is an important way for birds to communicate with one another. Voice imitating means a bird is imitating the crying or voice of other birds. The biological meaning of voice imitating is not clear even now and it is highly probable that it will take a long time for people to discover the mystery.