Why Do Birds Sing? Territory, Mates and a Learned Skill

Ornithologists separate calls from song. Calls are short, simple and mostly inborn: the alarm note that empties a hedge when a cat appears, the contact chirp that keeps a flock together, the begging of chicks. Both sexes use them all year. Song is longer and more structured, usually produced by males in the breeding season, and in the roughly four thousand species of true songbirds it is learned rather than inherited.

The two functions of song are territory and courtship, and they can be told apart by who is listening. A male that sings at dawn from a high perch, and sings harder when a rival approaches, is defending ground. A male that sings quietly and continuously near a female is courting. Playback experiments make the point: a loudspeaker playing a male's song in an empty territory keeps other males out for as long as it is running, and a female hearing a recorded song will approach the speaker.

A young songbird raised in silence grows up producing a crude, structureless version of its species' song, which shows that the rough outline is inherited but the details are not. Under normal conditions a chick hears adult males singing during a sensitive period in its first weeks, stores the sound, and months later begins to practise, producing a wandering, quiet babble that gradually converges on the memorised model. Peter Marler's experiments with white-crowned sparrows in the 1960s established this, and the parallel with how human children acquire speech has been studied ever since.

Learning has consequences. Songs vary from place to place, so a chaffinch in one valley sings a recognisably different dialect from one twenty miles away, and females often prefer the local version. It also allows song to become a test. A male that has learned a large, accurate and complex repertoire has shown that he was healthy and well fed during the months when learning was possible, and females in many species choose accordingly. The nightingale's repertoire runs to around two hundred distinct phrases.

Birds sing not with a larynx but with a syrinx, an organ at the base of the windpipe where it forks towards the two lungs. Each branch has its own set of membranes and muscles, so a bird can produce two independent notes at once, one from each side, and switch between them within a millisecond. That is how a wood thrush harmonises with itself and how a canary sustains a trill for far longer than a single breath would allow, by taking tiny breaths between notes too fast to hear.

The control comes from a set of brain regions found only in birds that learn song, which grow larger every spring under the influence of hormones and shrink again in autumn, one of the first clear demonstrations that adult brains can generate new neurons. Zebra finches, which learn a single song from their father and keep it for life, are now a standard laboratory animal for studying how the brain learns any sequenced skill.

Birds sing most in the half hour before and after sunrise, and several explanations have evidence behind them. The air is still and cool, so sound travels further with less distortion. It is too dark to forage and the insects are not yet active, so the time is cheap. A male that has survived the night is announcing the fact, and a territory whose owner falls silent at dawn is quickly claimed. The chorus is loudest in spring, when territories are being set and pairs formed, and fades once the young hatch and singing gives way to feeding.

The picture of song as a male performance comes from the temperate north, where most early research was done. In the tropics, where many species hold territories all year, females sing too, and pairs of some wrens and antbirds sing coordinated duets so precise that they sound like one bird. Recent surveys find that female song is the ancestral condition in songbirds and was lost in many northern species rather than never evolving.

Mimicry is the other surprise. Starlings, mockingbirds and marsh warblers weave the calls of other species into their own song, and the Australian lyrebird reproduces chainsaws, camera shutters and car alarms with unsettling accuracy. The point seems to be the same as elsewhere: the more a male can learn and remember, the better a mate he is judged to be. Song, in short:

• •Claims and holds a territory without a fight
• •Advertises a male's condition to females, who compare
• •Is learned from adults in a sensitive period, producing local dialects
• •Is produced by a two-sided syrinx that can sing two notes at once
• •Peaks at dawn in spring, when sound carries and foraging is impossible

Birds sing to hold territory and to attract and keep mates, and in songbirds the song is learned from adults during a sensitive period, which lets it vary by region and serve as an honest test of a male's quality. The dawn chorus happens when sound carries best and time is cheapest, and the whole system runs on a two-sided voice box and a brain that rebuilds itself every spring.