Each spring, the quiet of early morning suddenly fills with birdsong. The famous dawn chorus may sound spontaneous, but inside many birds a remarkable change has already taken place.

As daylight lengthens toward spring, hormones trigger growth in a part of the brain that controls song. Scientists call this region the HVC, the High Vocal Center. In some species, it can become 30 to 70 percent larger during the breeding season, allowing males to produce longer and more complex songs used to attract mates and defend territory.

The chorus itself also follows a quiet order.

Birds that can see well in dim light tend to begin singing first. In many places, robins are among the earliest singers, sometimes starting nearly an hour before sunrise. Soon chickadees, cardinals and sparrows join in, followed by finches and other songbirds as the sky grows brighter.

Early morning conditions help as well. In the still air before the ground warms, birdsong travels farther across the landscape.The result is one of spring’s most familiar sounds, a chorus powered not just by warmer weather, but by a seasonal change inside the brain.