From an interview with David Rothenberg, author of _Why Birds Sing_:
Q. In all your travels, did you ever learn why birds sing?
A. I think they sing for the same reason people do: because they can. It's part of their essence. Birds sing because that's what they evolved to do. They must.
The science on this is inconclusive. Biologists generally say that birds sing to protect territory and attract mates. And this is true of many species, though not always. It's not always clear that the bird that sings better has more mating success.
According to a study by Peter Merritt, a mockingbird that had the best song had neither the most mating success nor the best territory. The Australian lyrebird, which has a really complex song, sings for hours when there are few females in the area to even hear him.
I was fascinated to read about some researchers, who were studying the clicking sounds made by sperm whales - Moby-Dick. They couldn't figure out how the sounds fit together. So they called in an African drum master and he said: "Oh, there's a kind of a guiding rhythm there, and they do seem to be listening to each other. I really hear a pattern." He heard something the scientists didn't.
Q. Did you learn anything about bird-song scientists as you did your research?
A. I learned they are as territorial as they birds they study. When you go among them, one of them will claim: "I discovered this. This theory is mine!" From what I noticed, they didn't seem to spend a lot of time reading each other's papers or trying to duplicate someone else's results. Of course, the same can be said for philosophers, and musicians.