For the Birds

Maya Angelou may have know why the caged bird sings, but I wonder why the birds in the trees behind my house are singing at 4:30 in the morning.  The sound is pleasant, I don't mind hearing them, but why?  They can't be feeding.  It's still mostly dark.  It doesn't seem like mating calls either.  Aren't they done with that, for now? Most of the  calls sound cheerful and happy, though there is that kind of mournful cooing sound.....