Volcanic islands represent ideal laboratories for the study of ecology and living forms, for they permit flora and fauna to develop in almost complete isolation. From their origin as sterile masses of lava, over millennia they gradually develop topsoil; then seeds—borne by sea birds, by floating debris, or by humans on brief stops—begin to grow and develop. Once plant communities have been established, they form the basis for animal populations of marine mammals, pelagic birds, or insects and passerine birds blown off migratory courses by storms. A few small land vertebrates, depending on ocean currents, may arrive on floating rafts of vegetation. Once animal life has secured a foothold, it must cope with a climate, terrain, and ecology very different from those of its original habitat. In response, it adapts to the new conditions, over time developing into new and hitherto unknown species. Among the numerous volcanic islands of the world, the most famous are the Galápagos chain off the coast of Ecuador, which Charles Darwin visited in 1835 as a naturalist aboard HMS Beagle.
During his stops among the islands, Darwin collected specimens of flora and fauna for further study in England, as he had done in other locations on the voyage. The collected species included several finches of a kind he had never seen; all were similar in color but variable in size and behavior. Most striking to Darwin were the varying sizes of their beaks, suggesting different feeding patterns, though at first he saw little scientific significance in that fact. After returning to England, he posted their skins to the ornithologist John Gould in London and was surprised by his verdict that the finches belonged to previously unknown species. In time, the fourteen species of finches inhabiting the islands became the most studied group of birds in the world. Once it had been determined that they were all descended from a single species, numerous scientists have attempted to explain how this differentiation occurred. Since Darwin’s time, one of the native Galápagos species, the largest and the weakest flyer among them, has become extinct, but thirteen species remain on the islands.
Jonathan Weiner’s book is primarily an account of one such study of Galápagos finches. Despite its extensive scientific bibliography, it is not a scientific report but rather a narrative of the project of Peter and Rosemary Grant, who have led an ongoing study from Princeton University for the past twenty-one years. They have centered their study upon the ground-feeding finches indigenous to one small uninhabited island, Daphne Major, near the center of the Galápagos chain. While the ground feeders comprise six species, Daphne Major has only three or four species in any numbers. Lacking a freshwater source, the island offers researchers only primitive, stark living conditions. Its forbidding nature, however, ensures that life there has developed without human interference, a fact no longer true of the larger neighboring islands.
Hardly more hospitable to birds than to humankind, Daphne Major’s climate consists of a wet and a dry season. If seasonal rains are adequate, the finch population reaches two thousand or somewhat over that, but during the subsequent dry season, the population declines drastically to as few as three hundred during a severe drought. Because of the limited population of finches on Daphne Major and their relative tameness, the Grants have been able to capture and band every bird. Since nests are easily located amid the rather sparse vegetation, unfledged chicks are banded while they are still in the nest. The bands are designed so that individuals within the population can be identified by sight. Rarely, outside a confined area, could a researcher find a limited and easily accessible avian population for a study of this kind.
Unlike earlier researchers, the Grants have taken care to do as little harm to the finches as possible during their long period of study. Mist nets have long since replaced shooting as a means of capturing birds for study (during the early twentieth century thousands of Galápagos finches were shot for scientific study). The procedures used today enable researchers to compile adequate data while interfering little with the daily life of the birds.
Data obtained by the Grants include measurements of beak size, body weight, and wing span as well as records on reproduction and longevity of each bird. In addition, blood samples from individual birds are preserved for further study of alterations in DNA. As is usual in such studies, diets are determined through examination of the intestinal contents of dead birds. Over the twenty-one-year period, the Grants have accumulated data on eighteen thousand individuals. The sheer volume of data is manageable only through computers, and much of their time back at Princeton is spent analyzing the amassed...
(The entire section is 1998 words.)