That burst of birdsong at first light is called the dawn chorus, and it is one of the most reliable rhythms in nature. There are a few good reasons birds save so much energy for the early morning.
Sound travels best at dawn
In the still, cool air of early morning — before wind, traffic and daytime bustle pick up — sound carries much further. A bird singing at dawn can be heard across a far wider area, so its message reaches more mates and rivals for the same effort.
It's too dark to do much else
At first light it is often too dim to hunt for insects or seeds effectively, so singing is a productive use of time that would otherwise be wasted. As the light grows and feeding becomes easier, the chorus tends to quieten and the birds get on with breakfast.
Claiming territory and showing strength
Singing loudly at dawn also announces who survived the night and re-stakes each bird's claim to its territory and mate. A strong dawn performance signals a fit, healthy bird. This is why the dawn chorus is loudest in spring, during the breeding season.