Andrew Skiok

There's a lovely nuance to this question.

Some birds, not all.

Waterbirds, ducks, moorhens, geese and so on, are often quite vocal at night.

And as these are older lineages of birds that evolved earlier,

I speculate that birdsong originated in the dark.

It's only later in evolutionary time that songbirds, primarily, plus pigeons, parrots and others, have become daytime communicators.

So why do these groups sing so noticeably at dawn?

Part of it seems to be that daybreak is often the coolest and least windy part of the day, and as sound travels farthest in cool conditions, it's a good time to communicate across distance.

But more significantly, birds have evolved to sing at dawn because they use communication to negotiate their essential relationships.

these include attracting and bonding with a mate acknowledging their neighbours and respective home rangers particularly in the breeding season affirming communities of belonging with others of their kind and finding their place among other species within listening distance

They achieve this with formal patterns of singing behaviour and repertoire, all timed to utilise the optimal acoustic conditions of dawn.

With sunrise, they may initially fall quite quiet, but as the day warms, they'll pick up with vocalisations which achieve other purposes, such as maintaining foraging flocks, warning of dangers, or just keeping in touch with each other as they go about their day.

And if the wind rises by late morning and acoustic conditions deteriorate further, birds will often fall progressively silent.

So the answer is in how birds utilize atmospheric conditions to best achieve their essential acoustic communications.