(01 - 04) Read the given passage and answer the questions.
The song of birds is one of the sweetest sounds in nature. Sometimes when we are out in the country and we hear birds singing, it seems to us they are calling back and forth, that they are telling one another something. The fact is that birds do communicate with one another, just as many other animals do. Of course, at times the sounds birds make are more expressions of joy, just as we may make cries of 'oh!' and 'ah!'. But for the most part, the sounds that birds make are attempts at communication. A mother hen makes sounds that warn her children of danger and cause them to crouch down motionless. Then she gives another call, which collects them together. When wild birds migrate at night, they cry out. These cries may keep the birds together and help lost ones return to the flock. But the language of birds is different from the language we use. We use words to express ideas and these words have to be learned. Birds don't learn their language. It is an inborn instinct for them. In one experiment, for example, Chicks we kept away, from cocks and hens so they could not bear the sounds they made. Yet, when they grew up, they were able to make those sounds just as well as chicks that bad grown up with cocks and hens! This does not mean that birds can't learn how to sing. In fact, some birds can learn the songs of other birds. This is how the mockingbird gets its name.
01. When wild bird's migrater at night, they cry out: