Why do birds sing so loudly in the morning?

The dawn chorus concentrates in the hour around sunrise for several reinforcing reasons. Cool, calm morning air with little turbulence transmits sound substantially farther than warm midday air, so a song broadcast at dawn reaches more listeners for the same effort. Light levels are too low for efficient foraging on insects, which makes early morning a low-opportunity-cost time to spend on something other than eating. And singing hard immediately after surviving a cold night without food is an honest signal of condition, difficult for a weak bird to fake, which is exactly the kind of costly signal that sexual selection favors.

The chorus is mostly males, and it does two jobs simultaneously: warning rival males off a territory and advertising to females, with peak intensity during breeding season. Species enter in a fairly consistent order, with larger-eyed species that see better in low light generally starting earlier. Urban noise has measurably changed the behavior, with studies documenting city birds singing at higher pitches to cut through low-frequency traffic noise, singing earlier to beat the rush hour, and in some cases singing at night near artificial light. Nightingales in noisy areas have been recorded singing loud enough to exceed workplace noise regulations. The chorus is also a useful ecological indicator, since its timing and composition shift measurably with climate and habitat change.