Sunday, September 3, 2000		Nature

THE songs of birds are their way of communicating with each other to warn of danger, to call a mate or simply to keep in touch. It is generally the male who sings to attract a mate and to defend his territory, but in some species duetting is very common and boubou shrike is an expert in this art. It builds up an elaborate song from the contributions of male and female both. The songs of both the sexes are so well coordinated and finely integrated that they often sound like the song of just one bird. If one of the pair is absent, the lone bird has been observed singing both the parts. How such a fine coordination is achieved is a mystery.

Duetting birds live almost exclusively in the places with dense vegetation or in tropical forests where they often cannot see each other, therefore they depend heavily on sound. Most of these birds are monogamous (pairing for life) and retain their territories for many years. It is believed that in such a situation sound reinforcement is important for establishing and reinforcing the relationship between both the sexes and proclaiming the territory.

Shrikes are also known for their curious habit of impaling their prey on thorns or sharp twigs making it both easier to eat and to store. These larders may contain the impaled corpses of many animals, including insects, small birds, frogs, lizards and small rodents. In some areas these birds have switched over to a more modern material, using barbed wires in place of natural impaling implements. Because of the bird’s strange and macabre habit it is also called "Butcher bird".

As far as bird songs or bird communication is concerned it is a very complicated and interesting study. The familiar ‘too-whit, too-whoo’ of the tawny owl, often mistaken for single call, is in fact a duet between two birds — they may be male and female, or two males — who are checking each other’s whereabouts.

Black-capped chikadees of North America warn each other of a predator’s presence, then set up a Chorus of thin, ventriloquial notes. confused by the disembodied chirps, the predator often gives up the hunt. These chikadees have over 350 calls, each of which carries specialised information.

Some species of bird make use of low-frequency songs to attract a mate. The male sage grouse of North America, for example, uses an inflatable throat pouch to produce a

booming call. The male capercaillie of northern Europe has an apparently insignificant call, making it difficult to understand why the female should be attracted to it.

Shaking with effort, he emits a sequence of feeble pops and gurgles.

However, speeded-up recordings of the capercaillie’s call reveal that much of the sound the bird produces is to low in pitch for human ears. To make such a call normally, the bird would need a windpipe as long as a church organ bass pipe. But the capercaillie has a special tube in its throat, and the deep note is produced when air passes across the open end of this tube, in the way someone might blow across the top of a bottle.

Sometimes, however, birds’ trills and warbles seem unrelated to any particular activity, though they do tend to be uttered at specific times of the year. They may indicate aspects of bird life that have, as yet, no meaning for us.

The ability of a male American goldfinch to subtly alter his basic song shows that birds’ songs are not necessarily passed on unchanged from one generation to another. The birds breed late in summer, and during the breeding season this striking yellow male finch flies off to find food for his mate while she incubates the eggs. While he is airborne he makes his ‘flight’ call, a complex arrangements of notes, to which she responds.

As the season progresses, the male finch’s call becomes slightly modified, imitating that made by the female. Their similiar-sounding calls then identify the pair to each other throughout the breeding season. Should the female die or move away, the male can easily pick up a new female’s call, allowing him to remain a successful breeder throughout his life.