Birds don't all sing at once. They take turns, in a fixed order, starting an hour before sunrise. The chorus is a clock.

The order

Robins and thrushes start first, in the dark. Then warblers and finches join as the light comes up. By full sun, it's over.

Why it matters

Learn the order and you can tell the time by the birds. More usefully, the first hour is when the rare ones sing — be out before the coffee shops.