Why do birds sing, and who is listening?

Birds sing mainly to do two jobs: win a mate and defend a territory. Ornithologists separate songs from calls. Songs are longer and more complex and are tied to territory, courtship and mating, while calls do things like sound an alarm or keep a flock in touch.

For mating, scientists hypothesize that song evolved through sexual selection, and experiments suggest that the quality of a song can be a good indicator of fitness. Parasites and diseases may affect features such as song rate, which makes song a reliable signal of health. A female listening to a male is, in effect, reading a health report.

For territory, birds use song to negotiate their boundaries. Because song may reveal quality, rivals may be able to size each other up and avoid an energetically costly fight. The singer is saying: this ground is taken, and I am strong enough to keep it.

The instrument is the syrinx, a bony structure at the bottom of the windpipe, unlike the larynx at the top of ours. A bird controls pitch by changing the tension on its membranes, and pitch and volume by the force of its breath. It can also control the two sides of the windpipe independently, so some species can produce two notes at once.

Some of the details are still a mystery. In a 2023 study in southern Germany, male blue tits sang at high rates while their partners were still in the nest box at dawn, and stopped as soon as the females joined them. Scientists could not yet say why males sing more when females are absent.