Birdsong or: "Without music the world would be a mistake..."

...says Dickie Nietzche, somewhere. My question comes from listening to the BBC World Service a few weeks ago. One of their correspondents was investigating the evolutionary advantages conferred by musical abilities, ie why has birdsong evolved the way it has? The chap raised many interesting points, and pursued many religious/philosophical implications, but ultimately could only suggest that female birds are attracted by variety and complexity in male birdsong because it indicates what boils down to "good hearing" on the part of the singer, and thus the ability to anticipate and evade noisy predators, making him a fitter potential father. I didn't find this conclusion very convincing, but couldn't come up with anything of my own, so I thought I would throw it open to the ILM maseev. (Extra points will be given for: referring to the Carpenters, the myth of Orpheus and evolutionary psychology. Please show your working)