The Bengalese finch is a very interesting little bird. We know that it was only domesticated 250 years ago (from the wild white-rumped munia), but while the breeders have been developing its plumage, the birds themselves have been developing their songs, so that they now have a complex and beautiful song, compared to the munia’s plain and simple calls. Only the males sing, and it seems that the females select their mates on the basis of the quality of their song and tone, and the nicer the song, the more young they produce.
There is a video of this here http://www.youtube.com/watch?v=JnNnNumuuVk
This simple story has a number of very interesting implications