Why do males sing and females do not?
Scientists believe that the level of testosterone is responsible for song production. (Click here to learn more about testosterone). In the cross-section of the bird brains, you may have noticed the numbers within the pink circles. These numberes represent the percentage of cells that incorporate testosterone.
Upon exposure to the hormone, the size of the structures important for singing become larger, which in turn results in an increased amount of singing. As male birds grow and their developing brains are exposed to testosterone, the important structures mentioned previously are organized for song production.
In addition, as you've probably noticed, birds tend to sing more in the spring than in the fall. In male songbirds, this fluctuation of singing is correlated with seasonal fluctuations of testosterone levels, such that there are greater levels of testosterone present during the spring than in the fall. So, the nuclei grow in the spring as the bird starts singing and courting mates, then in the fall the nuclei shrink again and singing declines.
(images thanks to this page of bird pictures)