Do anyone know why the caged bird sings?

We all hear them early in the morning or afternoon, those simple chirp or musical notes created by birds. Birds use thier songs to communicate different messages with each other. Most birdsongs are created by the males, usually they sing to notify or attract female birds of his kind and signalling he has found a territory for her to mate and create a family on that location. He also sings to tell other males to keep out. So its natural that even a caged bird sings, for he might be looking for a mate, signalling other birds to keep out of his familiar territory or notifying other birds of his kind that he is there.