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| Acknowledgements | xi | ||||
| Introduction | 1 | (6) | |||
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| Appendix A: List of Common and Scientific Names | 151 | (5) | |||
| Appendix B: Supplementary Tables | 156 | (19) | |||
| Notes | 175 | (26) | |||
| References | 201 | (22) | |||
| Index | 223 |
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Chapter One
The Moa's Legacy
New Zealand's moas are no longer alive, but they molded the anatomy and behavior of New Zealand's surviving species. Their ghosts still hang over the New Zealand landscape.
Jared Diamond, "Bob Dylan and Moas' Ghosts"
The present-day flora and fauna of New Zealand bear little resemblance to those that made up the community in which keas originally evolved. Nearly half of all the species that thrived in New Zealand a thousand years ago vanished in one of the largest waves of extinctions in history. This "biological holocaust" began with the first arrival of humans on the islands and has continued up until recent times. A new set of actors has now taken over the ecological stage, filling roles that had been left vacant by the original occupants. To understand the biology of the kea, and the factors that influenced its evolution, we must begin by reconstructing the prehuman environment of the islands and repopulating it with the full range of original inhabitants. We must, in short, tell a ghost story.
WHEN THE MOAS REIGNED
Our story begins on the South Island about eight thousand years ago, when New Zealand first emerged in its present form. Temperate rainforests extended in all directions in a virtually unbroken canopy, occupying the western and eastern slopes of the Southern Alps up to about 1,200 m elevation. The highest tier consisted of Nothofagus, the southern beech, either in uniform stands or mixed with conifers and broad-leaved trees. Below were forests of podocarps, a group of conifers peculiar to the Southern Hemisphere, along with tree ferns and many different kinds of broad-leaved trees. In the northern parts of the South Island grew great forests of kauri, the largest and oldest trees in New Zealand. Only in the drier areas along the eastern plains and above tree line in the Alps did the forest give way to grassland and scrub.
This pristine wilderness had been isolated from other continents since the time of the dinosaurs. Instead of cattle, deer, and rabbits, New Zealand's grazers and browsers were all large, flightless birds. Gigantic moas, abundant on the South Island, were the most common browsing animal in the archipelago. There were eleven species of moas in six genera; one species included perhaps the tallest birds that ever lived. Similar in appearance to ostriches or emus, moas were adapted to a range of different forest and grassland habitats. Like bison in the American West, they were the keystone of the ecosystem, shaping the flora and the landscape of New Zealand and dominating all other groups by their weight and numbers.
Moas were found throughout the South Island, including the mountain forests, a habitat they must have shared with keas. Megalapteryx didinus, the most common moa on the South Island, was probably a mountain specialist, because the largest accumulations of its bones are from subalpine sites. Bones of larger moas, such as Pachyornis and Dinornis, have also been found in subalpine deposits. Dinornis, which were about the size of an elk, probably browsed on twigs, leaves, and fruit, occasionally taking fallen fruit from the forest floor. Smaller moas like Megalapteryx, which were about the size of an adult deer, may have been more omnivorous, including large invertebrates and insects in their diet. Moa chicks of all species may have fed mainly on insects and other small invertebrates. An analysis of moa gizzards suggests that moas ate at least some of the same plant foods as keas. For example, Dinornis seems to have fed heavily on the bright orange coprosma berries that are a staple in the kea's diet.
Among the most common grazers in the moa's kingdom were takahes and kakapos, birds that are extremely rare today. Takahes are large flightless gallinules with heavy, shearing bills that give them the appearance of small blue dinosaurs. They dig up grasses and then hold them in one foot while delicately peeling away the fibrous outer layers. The lumbering kakapo, a flightless nocturnal parrot the size of a turkey, was one of the most abundant birds on the South Island in the moa's day. Kakapos dig up roots and bulbs, peel bark and foliage from trees, and nip off buds, fruits, and shoots of low-growing shrubs. They also strip the seeds from grass and chew the blades, which are left dangling in a characteristic wad from the leaf end.
Many other, smaller, animals filled niches in the kingdom of the moas. Wetas, which are giant forest crickets, were then extremely common. They behaved much like mice, feeding on wood and leaves at night and leaping long distances when startled. They kept company in the underbrush with a diverse group of tiny, flighted and flightless New Zealand wrens, members of a unique family. A variety of smaller insects and worms, frogs, skinks, geckos, and giant land snails were common on the forest floor. These creatures probably served as food for tuataras, lizard-like reptiles the size of an iguana. Tuataras are the last remaining representatives of an ancient order of reptiles; they possess, among other odd features, a third, vestigial eye in the middle of their foreheads.
A large array of birds fed on insects in addition to fruit and nectar. Many of these species belonged to bird families with no clear relatives anywhere else in the world. The New Zealand wattlebirds included the oriole-like saddlebacks and the large, gray kokakos. The most spectacular wattlebird, however, was the huia, whose bill size differed between the sexes to a greater degree than that of any other bird species: the female's bill was nearly one and a half times as long as the male's. Wattlebirds fed mainly in the trees, while piopios, another species of exclusively New Zealand birds, searched for food on the ground. The underbrush was also home to nocturnal kiwis, with their long, probing bills; wekas, which are hen-sized flightless rails; and owlet-nightjars, which were distant flightless relatives of the whippoorwill and nighthawk.
In the daytime small insectivorous birds foraged in the foliage, among them warblerlike birds, such as whiteheads and yellowheads, and more robust species similar to chickadees, such as New Zealand robins and fantails. The nectar in flowering trees attracted stitchbirds, bellbirds, and tuis, members of a unique group of honey-eaters. Also fond of nectar are New Zealand parrots, particularly the kakarikis, which are New Zealand parakeets, and the kaka, the forest cousin of the kea; both go back to the time of the moas. One of the most beautiful birds in the moa's forest was the New Zealand pigeon, with its striking iridescent green and white plumage. New Zealand pigeons are consummate frugivores, putting away large volumes of fruits and leaves.
There were many carnivorous birds as well. Some were nocturnal predators, such as laughing owls and moreporks, both fairly small owls that mostly preyed on insects. Moreporks, however, do apparently eat nestlings of other birds, and perhaps the now-extinct laughing owls did so too. Flightless adzebills, about the size of a turkey, could easily have killed and eaten large lizards, as well as newly fledged chicks and eggs of other birds. But the main danger to adult birds would have been the larger raptors, which attacked from the air during daylight hours. Among them was one of the largest and most powerful birds of prey that ever lived, the Haast's eagle. Roughly the size of an Andean condor, it had a wingspan of nearly three meters. It may have been a "sit and wait" predator, observing from its high perch for long periods and then rapidly diving on its prey. Haast's eagles were certainly large enough to have fed on keas and may have been the sole predator capable of attacking the largest moas, whose carcasses would be consumed over several days.
Medium-sized aerial predators included the New Zealand goshawk and the New Zealand falcon, which would chase down other birds in flight. The now-extinct goshawk weighed about two to three times as much as a kea, while the falcon is comparable to the kea in size. These predators probably swooped from high branches and swerved through the forest in pursuit of their prey. The goshawk was surely a savage threat to a variety of midsized birds, including keas, kakas, kakapos, New Zealand pigeons, wekas, and New Zealand ravens. The falcon generally preys on smaller birds, though it will attack larger ones when the opportunity arises.
The presence of large predators opened a niche for scavengers. Today keas, harriers, and black-backed gulls all gather at carcasses and strip the remaining meat from the bones. In the moa's kingdom, however, the main scavenger was most likely the New Zealand raven. These birds were larger than keas and were abundant both in forest and in adjacent scrub regions. They were probably at least nominal omnivores, feeding on fruit as well as large insects, lizards, tuataras, and small birds. They undoubtedly scavenged carrion from moas killed by Haast's eagles and possibly preyed on the eggs and young of medium-sized birds, such as keas and kakas. They may even have fed on moa eggs and chicks. Raven and kea fossils have been found together in cave deposits, so the ranges of the two species overlapped, and both may have scavenged at the same carcasses.
THE KEA'S NICHE
At the time of the moas, keas probably fed much as they do today. Beech trees provide the bulk of their diet. Keas are more closely associated with forests of southern beech than with any other habitat. Their olive-green plumage blends perfectly with the somber shades of the beech foliage. The birds roost in the trees, nest in burrows in the depths of beech forest, and take shelter among the thick branches against predators or inclement weather. They also feed in the canopy and the second tier of beeches on whatever is seasonally available: buds, leaves, or nuts. When beechnuts are abundant, they forage for hours in the crowns of the trees. Keas pick the nuts off one by one, using their bills like a forceps, and then grind and crush the nuts between their lower bills and their hard palates.
The resources of the beech forests, however, are not enough to sustain keas. The low diversity in the understory of shrubs and forbs limits the amount of fruit, nectar, and edible foliage. Furthermore, beeches, like many other New Zealand plants, are mast-seeding, which means that they produce enormous volumes of nuts for one or two seasons and then very little for many years. From one year to the next, beech can be a very unreliable resource.
Keas must therefore seek out whatever food is seasonally abundant. In springtime they dig up large mountain daisies in the alpine grasslands, sometimes consuming the entire plant, roots and all. They also search at the edges of snow mounds, probing and digging around rocks for low-growing plants and insects. In summer keas forage in the alpine shrub habitat for fruit, foliage, seeds, and flowers. The birds relish the orange berries of coprosma bushes, of which there are over forty-five species. After beech trees, coprosma berries are probably their most important food source, but they also consume the red berries of the mountain totara. Keas feed readily from flowers, clinging to the branches of rata trees or mountain flax as they rapidly lap up the nectar and pollen. They also catch and eat huge numbers of grasshoppers, beetle grubs, and other insects.
In autumn keas spend much of their time feeding in the forest, where mountain beech buds and young leaves are plentiful. As in summer, they also forage on mountain daisy roots and coprosma berries. Roots, bulbs, stems, fruit, and seeds all continue to be important sources of food. The winter months of June to September, however, are the time of greatest mortality for keas, mainly due to starvation. The birds often feed below tree line on the forest floor, scrounging for remnants of fall berries. They also avidly seek animal fat and will fight vigorously to obtain it. They tear open carcasses to consume meat and internal organs; they scrape dried meat from bones, which they then open at one end to lick out the marrow.
IN THE COMPANY OF GHOSTS
Keas, then, are truly omnivorous, with a breadth of interest in plant and animal foods that roughly matches that of humans or coyotes in the American West. They consume an amazing array of foodstuffs, including as many as a hundred species of plants and animals (common examples in table 1). Very few of these resources, however, would have been exclusively theirs in the moa's kingdom. Keas had to compete with kakas for rainforest fruits and nectar, with kakapos and takahes for bulbs and succulent grasses, with moas for fruits and leaves, with the guild of insectivores for insects and grubs, and with ravens, harriers, and gulls for access to carrion. Only southern beech provided a staple that would have been relatively uncontested, perhaps because, apart from the rare years of high nut production, it was generally a marginal resource.
For all other foodstuffs the competition was probably fierce; many of the competitors were undoubtedly far more common than keas. Moas are found in every type of subfossil deposit in New Zealand. Moreover, because of their great size, even one moa would have consumed an enormous volume of leaves and fruits. Among the smaller birds kakas were abundant, and their range overlapped with the kea's at all but the highest altitudes. Even today, under protected conditions, their populations can easily be twenty times as dense as those of keas. In the moa's kingdom, kakapos and takahes would possibly have rivaled--in tonnage, if not in numbers--the rabbits that have since occupied their niche in the New Zealand fauna.
In a world of dietary specialists the kea survived as the ultimate generalist, feeding on almost anything that came its way and actively searching out alternative sources of food. Although kakas might open logs for beetle grubs more quickly, takahes might peel daisy bulbs more efficiently, and ravens might tear apart carcasses more vigorously, keas, when given the opportunity, could do all these things adequately.
With few exceptions, the species that once directly competed with keas or preyed on them have been eliminated from kea habitat or are either extinct or vastly reduced in numbers. The moas are gone, as are the eagles, goshawks, ravens, and adzebills. Kakas are restricted to remnant tracts of old-growth, native forest, and kakapos and takahes are now among the world's rarest birds. The kea lives on in a phantom community of predators, browsers, grazers, frugivores, and scavengers that once shaped its broad generalist strategy. It has survived in part because of just this ability to make use of whatever resources chance brings its way.
THE ORIGINS OF NEW ZEALAND
The kea is a product not only of its recent history but also of the conditions that made its evolution possible. These are rooted in the vast changes that occurred since New Zealand became a separate continent. In the age of dinosaurs, New Zealand was fused with the other southern continents into a single gigantic landmass, the supercontinent Gondwanaland. In this jigsaw puzzle of continents, New Zealand lay between the Pacific coast of western Antarctica and the east coast of Australia. Plants and animals dispersed throughout the region, moving easily through areas that today are separated by broad stretches of ocean. Forests of southern beech covered large parts of the supercontinent.
As Gondwanaland broke up about 80 million years ago, its component parts drifted off across the Southern Hemisphere, carrying the forests and their occupants with them. These floating continents bore southern beeches and lowland conifers, such as kauri and rimu, and many ferns and mosses. Today forests of southern beech provide a reminder of a common geographical bond. Trees related to New Zealand's southern beeches can be found in Chile, Tasmania, New Guinea, and New Caledonia.
Animals rode along with the trees. New Zealand's fauna contains many creatures that trace their ancestry to Gondwanaland forests, including frogs, skinks, geckos, land snails, some unusual spiders, and several groups of freshwater insects. The tuatara originated at this time, as did the wetas. The peripatus, an odd caterpillar-like animal that shares many of the attributes of arthropods and earthworms, is still found in New Zealand forests, as well as in South America and Africa, little changed from its Gondwanaland ancestors. The predecessors of the giant flightless moas may also have drifted along with the continents.
In the early stages of the continental separation, while the oceans were expanding between New Zealand, Australia, and New Caledonia, many organisms took advantage of the archipelagos that still tenuously connected the regions, working their way across by island-hopping. By the end of the age of dinosaurs, 65 million years ago, most of these islands were gone, and a new ocean, the Tasman Sea, stood between New Zealand and Australia. In spite of the now formidable distances, species continued to arrive, either blown on storm winds or drifting on rafts of floating vegetation. Even today, nearly 80 percent of the genera of higher plants found in New Zealand are shared with Australia.
As the continents became more isolated and their climates more distinctive, the flora and fauna of Australia and New Zealand began to diverge. Australia developed a diverse array of snakes and marsupial mammals that were apparently unable to cross the Tasman Sea. Aside from two species of bats, which must have arrived by air, New Zealand has no native land mammals and no reptiles other than lizards and tuataras. The absence of such large terrestrial predators was one of the major factors that shaped the strange and fragile ecology of the islands.
THE EVOLUTION OF THE KEA
Between 50 million and 25 million years ago, while mammals were diversifying and taking over the rest of the world, New Zealand was undergoing great tectonic upheavals. Mountains rose and fell, causing changes in sea level and severe fluctuations in climate. At the beginning of this period New Zealand was about 22 percent larger than it is today. By the end it had shrunk to only 18 percent of its present size. With this drastic reduction in land surface, species that had never before come in contact were forced to compete for limited resources, and a mass extinction of land organisms ensued. As the land rose again and stabilized, the lost species were gradually replaced by new groups invading from surrounding regions.
The ancestor of the three species of parrot in the genus Nestor --the kea; its brown cousin, the kaka; and their close relative, the Norfolk Island kaka--probably came from Australia. But the taxonomic evidence of a relationship between Nestor and the Australian parrots and parakeets is not definitive. It would have to be a very distant kinship, indicating a long history of separate evolution. Thus, the ancestral Nestor may have arrived in New Zealand as much as 20 million years ago. A forest-dwelling parrot, the "proto-kaka," is presumed to have lived in New Zealand about 15 million years ago, when the region consisted of a single, large island. At that point the geology of New Zealand remained comparatively quiet for about the next 10 million years.
At the start of the Pleistocene, about 2 million years ago, this stability was shattered. New Zealand entered a tumultuous period of sharply oscillating climatic conditions. As temperatures cooled, glaciers flowed out from the mountains and covered the land. During these glacial periods the forests retreated and sea levels dropped, causing smaller islands to join the mainland. When warm temperatures returned, during the interglacial periods, the glaciers receded, the forests spread, and sea levels rose, inundating low-lying areas and dissecting New Zealand again into many small islands, which would later merge in colder periods. As a result of this fluctuation, the contrast between northern and southern environments became extreme. In the south, at the last glacial maximum, about 22,000 to 14,000 years ago, a continuous chain of glaciers and ice sheets stretched over the Southern Alps. In the north, however, temperatures were milder and glaciation was relatively minor.
This geographic contrast, combined with the repeated fragmentation and reunification of the habitat, is a classic recipe for generating new species. When a continuous, interbreeding population is separated by climatic or geographic barriers that create distinctive habitats, the populations are subjected to different selective pressures and may evolve different features. When the barriers are subsequently removed, the accumulated differences may be sufficient to prevent the populations from freely interbreeding again; in other words, they will have become separate species. The longer the populations remain isolated from each other and the more extreme the habitat differences they experience, the more likely it is that they will diverge and become new species.
The proto-kaka may have diverged into new species sometime in the early Pleistocene. Several populations of this forest parrot were physically separated from one another by the subdivision of the islands as sea levels rose. The population in the more benign north became kakas, specializing in exploiting the fruits and insects of the rainforest. The population living in the harsher southern regions eventually became keas, developing the behavioral strategies and food preferences that would help them survive among the ice fields.
When keas first evolved, forests were sparse on the South Island. Pollen records suggest that all trees except southern beeches were rare. Beneath the glaciers and snow much of the South Island was covered in grassland and open herb fields. Many of the alpine communities of the South Island developed during this period, producing a flora of remarkable diversity and uniqueness. Over half the native plant species in New Zealand are restricted to the alpine regions, and many of these are found nowhere else on earth.
Evidence from fossils found in limestone caverns indicates that keas were quite common in the mid-Pleistocene. Their flexibility and cleverness, products of their evolution amid the unforgiving ice and snow, stood them in good stead in exploiting the erratic, patchy food supply of the cold grasslands. As the ice retreated, between 14,000 and 10,000 years ago, grassland gradually gave way to denser shrub land in the South Island, but there was still relatively little forest cover. Then, about 9,500 years ago, pollen records show a dramatic change. In less than a thousand years, forests of not only beech but also conifers and broad-leaved trees advanced to blanket most of the South Island.
Coincidentally, about nine thousand years ago, something led to a severe reduction in the kea population. The fossil records suggest that at about the time when the forests of the South Island were expanding, keas were becoming rare. What happened is far from clear. The increase of the forests should not, in itself, have threatened the kea. Although the birds are primarily adapted to alpine conditions, they readily make use of the abundant resources available in lowland forests. In some fossil caves keas continued as a major component of the fauna even after the shift from subalpine and montane to lowland forest conditions. Thus, habitat modification cannot have been solely responsible for the species' subsequent abrupt decline.
Clues to the mystery may lie instead in the company of competitors that the kea came to face in a milder and more hospitable climate. When the forests returned, they presumably brought with them the characteristic fauna of the moa's kingdom, including the forest-adapted kaka. Confronted by these foraging specialists, keas would have been forced back into a receding alpine habitat, reduced to a sparse and less rewarding spectrum of foods. As a result, their numbers dwindled. But even so, their shrewdness and resiliency allowed some of them to survive, making their living in the hostile alpine environment or in the neglected corners and interstices of the forest, where they would scrounge bits that the specialists had overlooked or not yet gotten around to. An uncommon species of harsh and marginal habitats, keas might have continued indefinitely in this fashion had it not been for the arrival of humans in New Zealand.
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