In the course of evolution, islands have often served as nature's laboratories where isolated populations are permitted or forced to follow unusual evolutionary paths. One path followed in several locations was the tendency of birds to evolve to unusual size and to lose the capacity for flight when shielded from predators. The dodo (Raphus cucullatus) of Mauritius and the lesser known solitaire (Pezophaps solitaria) of Rodriguez are probably the best known and most recent examples of extinct giant, flightless island birds. In other birds, such as the moas of New Zealand and the elephant birds (Aepyornis maximus and Mulleror-nisbetsilei) of Madagascar, the sizes reached were even more extreme. These giant, flightless birds share two characteristics: their vulnerability to introduced predators—the greatest of them all being man—and the fact that with very few exceptions they are now extinct.
When Europeans first landed on Mauritius, they were dismayed to find a lack of desirable game. The only large animals to be found were the ungainly dodos. These pigeon relatives were 15 to 20 kg in weight, with males larger than females. Some accounts also suggested that the birds were sexually dimorphic in plumage and coloration. Both the dodo and the solitaire are thought to have been primarily frugivorous, and it has even been hypothesized that the dodo's digestive system played a critical role in the germination of the seeds of a now endangered tree (Siderox-ylon grandiflorum) (Temple, 1977). Invertebrates might also have been consumed as a component of the birds' diet (Livezey, 1993). The sailors remarked about the birds' seasonal accumulation of body fat. This ability to put on great amounts of fat in times of plenty may have been important in sustaining them through the leaner months. These observers also noted that dodos were territorial, and that parent birds were vigilant guards of their nests and young. Significant parental investment was justified, since each nest contained a single egg. This reproductive strategy was suitable in the absence of predators, but it did not allow for compensation when the equilibrium of the population was disrupted. The early naturalists were harsh critics of the quality of dodo meat; nevertheless they continued hunting them until the last had been consumed in the early eighteenth century. The extinction of the dodo (and solitaire) was particularly significant, because it was the first case during historical times in which humans recognized the tragic results of their overharvesting.
In addition to human predation, habitat destruction has also been identified as a major factor in the extinction of several giant birds. The moas (families Dinornithidae and Anom-alopterygidae) of New Zealand, elephant birds of Madagascar (Aepyornis maximus and Mullerornisbetsilei), and mihirung (Genyornis newtoni) of Australia shared this fate.
Prior to the arrival of humans approximately 1,000 years ago, several species of moa inhabited the heavily forested islands of New Zealand. Among the two families of moas were species ranging in weight from 20 to more than 200 kg. These enormous birds were herbivores, and they occupied the niches filled in other places by mammals such as the ungulates. Although the moas were previously thought to have been grazers, analyses of well-preserved specimens indicate that they were in fact browsers. Twigs, leaves, fruits, and seeds of woody plants have been found in the gizzards of some specimens. Large quantities of stones that aided the digestion of tough plant material were also found among the gizzard contents. Differences in beak morphology and body size suggest that the various species were adapted to particular feeding niches, and defense mechanisms of certain plants hint at coevolutionary relationships between moas and some plant species (Cooper et al., 1993).
Little is known about the habits and life histories of the moas, except what can be inferred from their anatomy and the contexts in which remains have been found. Maori folklore greatly romanticized the moas but contributed little factual information. Prior to the arrival of humans, the islands of New Zealand were heavily forested, and moas are thought to have been primarily forest-dwelling birds. Analysis of moa remains indicates that certain species occupied distinctive habitats ranging from the coastal lowlands to montane forests more than 1,000 m above sea level (ibid.). Some moa nests, each containing a single egg, have been found in caves, and others have been found in open areas. Like the dodo, moas were probably long-lived, slow-growing birds with a low total lifetime reproductive potential (Anderson, 1989).
Moa species endured significant climatic and habitat changes caused by volcanic eruptions and periods of glaciation that undoubtedly had significant impact upon their populations. Nevertheless, significant numbers of the birds persisted, as demonstrated by the vast quantities of moa bones found in the refuse middens of the first human inhabitants. It appears that the new human inhabitants found the moas to be fairly easy prey, and, as the only large game, moas were aggressively hunted for food. Moa skins and bones were used to craft tools, ornaments, and garments. Nests were robbed, and the eggs became a source of both food and storage containers.
In addition to direct predation, humans also affected moa populations through deforestation and the introduction of other invasive species. Forest fires destroyed both the habitat and food supply of the moas. Introduced plants interfered with the regeneration of native plant communities and may not have been suitable as food for the moas. Rats (Rat-tus exulans) and dogs had a significant impact on the smaller terrestrial avifauna and may also have affected the moas, although probably to a lesser extent.
It is generally accepted that the extinction of the moas was a direct result of human impact. However, there is some controversy regarding the rate at which the extinctions occurred. Trotter's and McCulloch's (1984) assessment of the fossil and archaeological evidence indicates that peak periods of moa hunting occurred 800 years ago in the northern areas and 500 years ago in the southern areas, possibly in association with southward human population expansion. Another more recent analysis by Holdaway and Jacomb (2000) suggests that all species of moas were driven to extinction within 100 years of human arrival, because rates of predation overwhelmed the populations' regenerative capacity. All of the moa species were extinct by the time of the first European arrival in the late eighteenth century. Other flightless birds—such as a flightless goose (Cnemiornis calcitrans) and the giant rail (Aptornis otidiformis)—also fell victim to predation, and the giant eagle (Harpagornis moorei), a carrion eater, was probably a sec ondary victim of moa extinction (Trotter and McCulloch, 1984).
The dodo, solitaire, moa, and other giant birds shared similar evolutionary histories, including their common fate of extinction. Isolated populations of birds, in the absence of predators, were freed from the need to be light and swift. Evolution of large body size is often associated with a decreased metabolic rate, which in turn is associated with a greater ability to withstand extremes of temperature, the capacity to endure periods of fasting, and increased longevity (Livezey, 1993). In this situation, the capacity for flight became an unnecessary and metabolically costly extravagance, and so it was gradually abandoned. The absence of predators also allowed these slow-growing, long-lived birds to adopt a strategy of low total-lifetime reproductive output, in which a high degree of parental investment was devoted to small numbers of offspring (ibid.). In some cases, the birds were the only large terrestrial vertebrates, and so they followed a course of adaptive radiation to fill the otherwise unoccupied niches.
In the security of their island sanctuaries, giant birds flourished. On the island of Madagascar, eggs of the elephant birds (Aepyornis maximus, Mullerornisbetsilei) were deposited in such vast numbers that they can still be found quite readily despite the fact that the birds became extinct in the seventeenth century (Dewar, 1984). The mihirung (Genyornis newtoni) of Australia was represented in the fossil record for at least 50,000 years before its sudden extinction, coinciding with the arrival of humans (Miller et al., 1999). The success of this pattern of evolution is demonstrated by the many other extinct species of large, flightless island birds that are known, and others that are still being discovered by paleontologists.
Sadly, just as the pattern of evolution has been repeated, so too has the course of human
impact on naive environments. The characteristics of the giant, flightless island birds that-made them well adapted for their isolated life also made them extremely vulnerable to humans. When attacked, the birds were essentially defenseless. Long-legged birds such as some of the moas may have been able to kick, but that would have been little defense against men armed with spears. Some of the birds may have been adapted for running, but most of them were probably slow and burdened by their ponderous weight. For birds such as the short-legged dodo, kicking and running were beyond their ability. Indeed, accounts of dodo hunts suggest that they offered no struggle at all, perhaps because they lacked the instinct of fear. Young birds, with their prolonged incubation and altricial periods, were particularly vulnerable both to humans and to other predators. With few offspring produced, individual losses significantly affected the population (Livezey, 1993). The fact that other large birds such as ostriches and emus have survived may be in part because of their reproductive strategy of laying eggs in clutches and thereby decreasing the importance of a single offspring. Environmental changes caused by deforestation may have eliminated the food supplies of these specialized herbivores. Adaptive changes designed for life in a system at equilibrium had become the burden of species in the face of change.
See also: Alien Species, Birds; Coevolution; Extinction, Direct Causes of; Mass Extinction
Anderson, Atholl. 1989. Prodigious Birds: Moas andMoa Hunting in Prehistoric New Zealand. Cambridge: Cambridge University Press; Cooper, Alan, et al. 1993. "Evolution of the Moa and Their Effect on the New Zealand Flora." Trends in Ecology and Evolution 8, no. 12: 433-437; Dewar, Robert E. 1984. "Extinctions in Madagascar: The Loss of the Subfossil Fauna." In Quaternary Extinctions: A Prehistoric Revolution, edited by Paul S. Martin and Richard G. Klein, pp. 574-593. Tucson: University of Arizona Press; Holdaway, Richard N., and Chris Jacomb. 2000. "Rapid Extinction of the Moas (Aves: Dinornithiformes): Model, Test, and Implications." Science 287: 2250-2254; Livezey, Bradley C. 1993. "An Ecomorphological Review of the Dodo (Raphus cucullatus) and Solitaire (Pezophaps solitaria), Flightless Columbiformes of the Mascarene Islands." Journal of Zoology, London 230: 247-292; Miller, Gifford H., et al. 1999. "Pleistocene Extinction of Geny-ornis newtoni: Human Impact on Australian Megafauna." Science 283: 205-208; Temple, Stanley A. 1977. "Plant-Animal Mutualism: Co-evolution with the Dodo Leads to Near Extinction of Plant." Science 197: 885-886; Trotter, Michael M., and Beverly McCulloch. 1984. "Moas, Men, and Middens." In Quaternary Extinctions: A Prehistoric Revolution , edited by Paul S. Martin and Richard G. Klein, pp. 708-727. Tucson: University of Arizona Press.
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