From: Interview with Dr. Luis Baptista, ornithologist, New York Times, Tuesday, May 16:

Q: Why do birds sing?

They sing, for one thing, because the bachelor male wants to attract a wife. He also wants to show other males that he is pretty studly, that this is his territorhy and no one is going to cross the boundaries. A third reason is that the song induces the female bird's brain to send hormones: it makes her ovaries grow.

Q: What are the parallels between human music and bird music?
A: Some birds have tonal qualities like real instruments. The strawberry finch has beautiful single notes that come down the scale and sound like a flute. The diamond firetail from Australia sounds like a woodwind. The black palm cockatoo actually shapes a piece of a stick that it uses like a drunstick. It holds it with one foot and bangs it against a hollow log. Woodpecker seek out a nice log that makes the correct sound. Different birds use different pitches. One scholar discovered that the hermit thrush sings in the pentatonic scale used in Far Eastern Music; the canyon wren sings in the chromatic scale.

Birds can sing in canonical or sonata style. Very often they have variations on a theme, and eventually they come back to the original theme. They probably do it for the same reason human beings compose sonatas. Both humans and birds get bored with monotony. And to counter monotony you always have to do something new to keep the brain aroused.