Why daybreak but not twilight or in unison at some point in the day? And why does that know-it-all tweeter, the Mockingbird, sing at night?
Ok - I found a thread on this board that addresses the questions – I needed to use the word ‘dawn’ rather than ‘daybreak’ - ‘morning’
Sorry 
Some birds sing to mark out their territory - eg the kookaburra, which sings at dawn and dusk.
If a mockingbird is like a koel, (a migratory, night singing bird) then it might be trying to attract a mate.
Yes, the male mockingbird sings to attract a mate. The female is generally impressed with the male who knows the most sounds. An unmated mockingbird will sing the night away. We have been “treated” to the same male for 2 years now.
So birds sing to attract a mate at night? Is that a avian one-night stand (or would “one-night nesting” be more accurate)? Sounds a lot like the singles bars around here!
Well, if you’re yearning for silence, I’d suggest you go to the pet store and buy a female mockingbird to release in the backyard for him. (This in spite of the disapproval likely from this other thread “I What’s wrong with being a whore?”.
I think that singing at the first sight of dawn is just the right and natural way of life.