Why do birds sing?

Male birds actually do most of the singing, primarily to stake
out their territory and to invite females of their species over
to mate. Females tend to select as mates those male birds who
sing the most. It is believed they do this not because they like
the quality of the singing, but because they have learned the
males who sing the most have the most food in their territory.
Since the male doesn't have to spend much time hunting for food,
it has more time to sing.