Why do small birds risk attracting predators by singing atop trees? Peter Slater tells Julia Hinde his theory, which he will flesh out in a forthcoming Darwin lecture
In a world teeming with sparrowhawks and other birds of prey, sitting atop a tree and singing your heart out is surely the most dangerous thing a small bird can do. Yet despite the dangers associated with such risky, attention-seeking behaviour, millions of birds perch on branches performing endless musical repertoires. Some birds even sing two separate sounds at once in a bid to attract attention.
So why do birds repeatedly commit such potentially suicidal acts and how do they learn the tunes they recite? Peter Slater, Kennedy professor of natural history at the University of St Andrews, has spent the past 20 years studying bird song, convinced that it provides a key to understanding many areas of animal behaviour. Slater, who will next month in Cambridge deliver one of this year's Darwin lectures, which are all based on the subject of sound, says that behind the often intricate repertoire of tweets and chirps is a complex interplay of sexual attraction, territorial defence, nature and nurture, that still poses mysteries for biologists today.
Production of bird sound is, according to Slater, quite unlike human sound because the syrinx, the bird's sound-producing organ, is low down the respiratory system, at the junction of the lungs and the trachea.
The syrinx in many birds is operated by a pair of muscles, but in the best and most complex singers, such as songbirds, the syrinx is operated by five or more muscles. The muscles alter the tension on two membranes on either side of the syrinx. As with a musical instrument the sound produced depends on the tension. The more taut the membranes, the more rapidly they vibrate and the higher pitched the sound. "This remarkable apparatus," says Slater, "accounts for the extraordinary speed and precision with which birds can sing."
"It also has a feature which gives them an edge on us when it comes to complexity," he adds. "Because there are two membranes, each with its own set of muscles, birds can produce two separate and harmonically unrelated sounds at the same time. I know a man who recorded a starling who imitated a gull with one side and a warbler with the other. It gives you two strings to your bow."
According to Slater, bird song, as opposed to short simple calls, is largely the domain of males and is mostly produced during the breeding season. However, the bird world, he stresses, is full of exceptions with, for example, female robins keen singers in winter.
Plenty of evidence exists, he says, to suggest that the role of song is partly to attract and stimulate females and partly to repel rival males - a kind of keep out signal. He notes too that the song produced by some species of males may differ for each of the two functions - with the longer and more elaborate song, or the greater repertoire of tunes, reserved for impressing the women.
When it comes to attracting females, Slater notes: "Not only does the song stimulate females of a number of species to display, but it has also been found in canaries to increase their nest building behaviour and the growth of their eggs."
Song learning, says Slater, is one of the clearest examples of the intricate interplay between nature and nurture in animals. While it may be possible, he believes, to teach a young male the song of another species, in the great majority of cases males sing only the songs of their own species. "While they learn their songs," says Slater, "there are constraints which effectively limit them to learning only those ones that are appropriate."