This is the nerdy answer. I used to work for US Fish and wildlife and the National Zoo doing research on birds. There was this wonderful guy named Gene (Morton, I think) who studied why they sing. They sing as early in the day (Oh-dark-thirty) to announce they are on their territory, attract females, and to challenge those that want to fight for both. They sing certain songs certain times of year because of day length and temperature. They are sorta hard wired. He also studied how they sing, which is even nerdier. Isn't it wonderful that someone could make a living having a passion about birds singing?