Academic Reading — All-Types Test 19

Reading passage

The Science of Birdsong A The dawn chorus that greets a spring morning can seem, to the casual listener, a pleasant but random jumble of sound. To an ornithologist, however, it is anything but random. Each burst of song carries information, and the birds producing it are engaged in a highly structured form of communication that has been refined over millions of years. Broadly, biologists distinguish between two categories of vocalisation. The first, known as calls, tends to be short and simple, and is used for immediate purposes such as warning of a predator or keeping a flock together. The second, song, is generally longer, more elaborate, and largely reserved for two tasks: defending a territory and attracting a mate. It is song, with its melodic complexity, that has drawn most scientific attention. B How does a bird produce such intricate music? The answer lies in an organ unique to birds called the syrinx, situated where the windpipe divides to enter the lungs. Unlike the human larynx, which sits higher in the throat, the syrinx has two separate sound sources, one on each branch. This arrangement allows some species to sing two different notes at once, in effect harmonising with themselves. Songbirds control the syrinx with remarkable precision, adjusting the tension of tiny membranes and the flow of air to shape each note. A wren weighing less than a small coin can pour out several hundred notes in the space of a few seconds, a feat of muscular coordination that continues to impress researchers. C Perhaps the most surprising discovery of the past century is that many birds are not born knowing their song; they learn it, much as a human child learns speech. Young songbirds pass through a critical early period during which they listen to and memorise the songs of adults around them, usually their father or near neighbours. Later, they begin to practise, producing a rambling, imperfect version known as subsong that gradually sharpens into the crisp adult performance. If a young bird is raised in silence, deprived of any model to copy, it will develop only a crude and abnormal song. This process of imitation and refinement has made birds valuable subjects for scientists interested in how any animal, including ourselves, acquires a complex skill. D Song also varies from place to place in ways that echo human language. In several well-studied species, populations living in different valleys or regions sing recognisably different versions of the same basic song. These regional variants, which researchers call dialects, arise because young birds copy the local adults rather than some universal template. A male sparrow raised in one district may therefore be marked out as a stranger the moment he sings in another. Females of some species appear to prefer the dialect of their birthplace, which may help keep populations distinct and, over long stretches of time, could even contribute to the formation of new species. E Why sing at all, given the effort and the risk of attracting predators? The prevailing explanation is that song advertises the quality of the singer. A male who can produce a long, complex and tirelessly repeated song is, in effect, demonstrating that he is well fed, healthy and free of parasites, and therefore a desirable partner or a formidable rival. Field experiments support this idea. When researchers played recordings of vigorous song from loudspeakers placed in an unoccupied territory, rival males were reluctant to intrude, evidently persuaded that the ground was already held by a strong occupant. Song, in short, is not decoration but hard information, honestly signalling something the listener cannot otherwise see. F For all that has been learned, much remains uncertain. Scientists still debate exactly how female birds assess the songs they hear, and why a few species maintain repertoires of dozens or even hundreds of distinct song types when a single reliable tune might seem to suffice. What is no longer in doubt is that birdsong, far from being idle noise, is a sophisticated system shaped by the twin pressures of competition and choice, and that in studying it we glimpse the deep principles governing communication throughout the natural world.