Mike found that young birds in isolation could only make some simple sound. Mike learned that a young bird has to hear other old birds’ song during the period before it is seven weeks old. Then there is a long time until the next spring before the bird begins to sing. And when it does start, it needs to practice before it finally gets the song right.
Further studies have proved that young birds do indeed need musical education. After several of the young birds raised in isolation were put together for about ten months, they learned from one another and finally could sing. The songs they sang, however, were quite different from the wild birds’. On the other hand, the young birds caught after their time was spent listening to the wild birds’ songs, knew how to sing.

Mike found the truth that（）