Researchers have discovered that a "day in the life" for an average bird consists of:

Singing:  9 hoursSleeping: 9 hoursEating/Misc.  6 hoursAfter observing and documenting the bird activity outside my home over the past few months, I'm guessing that once the brood is hatched, and there is a constant need to keep the gapes of nestlings and young birds full, there is a lot less singing and sleeping.