TACKLING STOATS Mohua

Forest and Bird, Issue 281, 1 August 1996, Page 21

TACKLING STOATS Mohua

Thanks to

and

for

their contributions to this article.

COLIN O'DONNELL

Elaine Murphy

Peter Dilks

Recent studies of mohua, or yellowhead, provide the first detailed evidence of the magnitude of stoat impacts on the long-term viability of a threatened endemic bird.

looks at this research and describes developments in the battle to control stoats In New Zealand forests as better control techniques become available.

IGH IN A HOLE in a red beech tree, a clutch of three hungry blue-grey mohua nestlings jostle for position awaiting the arrival of a parent with a juicy caterpillar or beetle. They hear a scratching noise outside the nest, but instead of their mother or father the sound is that of a stoat — the most efficient killer in the forest — that has heard the

chicks from some distance away. Within a few seconds the nestlings are all dead. Millions of years of mohua evolution in ancient New Zealand have left the species with no defence strategies to counter exotic predatory mammals. Since mammalian predators were introduced to this country, many bird species have become extinct or reduced to small populations on predator-free

islands. Kiore arrived with early Polynesians, and a further three rodent species were introduced by Europeans after 1770. Three mustelid carnivores — stoats, weasels and ferrets — were released in the 1880s in an unsuccessful attempt to control rabbits. Domestic cats also became wild and spread throughout the country. Early naturalists and ornithologists frequently commented on the devastating

effects of these mammalian arrivals on native forest birds, though most examples of predation were anecdotal. We now know that avian prey has been identified by researchers as a major part of the diets of stoats and weasels in New Zealand, and several detailed studies have found that a large proportion of forest bird nests can

be preyed upon by these animals as well as by rats. One study, by Phil Moors, found that eight native bird species at Kowhai Bush, Kaikoura, lost 70 percent of their nests to predators — with stoats and weasels responsible for more than three-quarters of these. Doug Flack recorded a similar

result for South Island robins in the same area: 55 percent of nests were preyed upon, with mustelids destroying 44 percent and rodents nine percent. Predation rates varied markedly between years, as did densities of mustelids. However, neither study determined whether this high level of predation

reduced the long-term viability of the bird populations. More recent studies of mohua, kaka and kakariki (yellow-crowned parakeet) confirm that stoats and probably rats are still having a major impact on mainland bird populations and that the process of decline is continuing.

The work with mohua provides the first quantified evidence of the magnitude of stoat impacts on the long-term viability of a threatened endemic bird. Similar threats also face the New Zealand forest parrots, kiwi, kereru, robin, Hutton’s shearwater, black stilt, takahe, and possibly other species such as our bats, kokako, and penguins. OUND ONLY IN the South Island, the mohua (Mohoua ochrocephala) is a small insectivorous bird which has disappeared from extensive areas of relatively unmodified forests since the arrival of Europeans. Historical records report that mohua were once among the most abundant and conspicuous forest birds in the South Island and Stewart Island and were present in most forest habitats of these islands — some 6.5 million hectares. Mohua now only occupy about a quarter of their historical range (see map), and are continuing to decline. Their main predators are stoats. During most mohua

breeding seasons stoats are uncommon and few nests are attacked. However, a heavy seeding of beech trees every four to six years produces a chain reaction that leads to high stoat numbers in forests the following summer. The increased food availability allows increased litter sizes and

survival of young, and insect, mouse and then stoat numbers increase dramatically. Stoat numbers can reach plague proportions. In two areas studied during several high seed years — the Hawdon Valley in Arthur’s Pass National Park, and the

Eglinton Valley in Fiordland — about 70 percent of mohua nests were preyed on, apparently by stoats, and about half of all nesting females disappeared. In populations with low productivity, the period between crashes is probably insufficient for mohua numbers to recover

fully from high levels of predation. In the Eglinton Valley, for example, five years after the last stoat plague, mohua numbers have still not reached their former levels. Mohua are more vulnerable to predation than most other forest birds. In fact as far as defence against stoats is concerned they do almost everything wrong. They nest in holes so that nest predators not only eat eggs and chicks but also incubating adults which are unable to escape. Only females incubate, so nest predation causes a biased sex ratio among adults. They also have long incubation and nestling periods during which they are vulnerable to predation. Chicks are very noisy on the nest, making them conspicuous targets for predators. Mohua nest later than most other forest birds and are still nesting when stoat numbers reach their summer peak. Most of these features are shared by other hole-nesting birds in New Zealand, unlike hole-nesters overseas which generally have well developed predator avoidance strategies. These findings led the Department of Conservation to undertake more research on methods for predator control. DoC’s "Hole-nesting Forest Birds and Predator Programme" has been investigating three fundamental questions: » are predators still having a significant impact on the long-term survival of forest bird populations?

» can we predict when predators will affect forest bird populations? » can we increase the productivity and health of forest bird populations by controlling predators? The programme, which has been running since 1990, has made considerable progress towards identifying the benefits of stoat control for native wildlife and

refining trapping and _ poisoning techniques so that cost-effective control can be achieved over large areas.

ETER DILKS, Graeme Elliott and myself have been studying the nesting success of mohua in areas where stoats were both trapped and left undisturbed. During the last stoat plague in 1990, a 50-hectare block of forest was trapped intensively. Sixty-two stoats were caught during the breeding season and 80 percent of mohua nests were successful. However, only 36 percent of nests weré successful in a similar-sized untrapped area and a high proportion of adult females were eaten by predators. Mohua pairs produced nearly twice as many young in the trapped area from fewer nests. In the two years following the stoat plague, nest success was higher than previously recorded in both trapped and untrapped areas, indicating that trapping in non-plague years is also beneficial. The success of this operation showed that intensive trapping can be effective at controlling predator numbers to a level that benefits threatened bird populations. This suggests that further development of stoat control techniques is warranted. The costs of not controlling stoats will lead to severe impacts on New Zealand wildlife communities in the longer term, and management of predator populations will be essential if mohua, and probably other wildlife populations, are to recover in the future.

Success was, in part, due to extensive trials of the effectiveness of trapping stoats using different bait types, tunnel designs and trap positions, during and following stoat plagues. The result is more effective prescriptions for control operations. DoC is now doing more research to develop and refine control techniques in order to sustainably control stoats over large areas. Current projects include the development of poisoning strategies for stoats, designing the most effective layout for traps or poison baits, and understanding the "flow-out" effect and potential benefits of controlling stoats in small areas on neighbouring forest. Trapping could still be improved by finding even more effective lures and tunnel designs. Mohua have a good potential for recovery if the factors that have caused their decline can be eliminated or reduced significantly. They lay up to four eggs, and are capable of raising two broods a year. DoC’s Mohua Recovery Programme is now up and running. The programme is important because it also addresses conservation problems being faced by many other endemic forest birds on the mainland. Successful mohua recovery has implications not only for other hole-nest-

ing species such as kaka, parakeets and robins, but for the whole forest bird community. The long-term goal of the programme is to maintain and enhance mohua populations throughout their present range and beyond, by halting and reversing the degradation of the forest ecosystem. Eleven key sites for initial protection have been identified and predator control programmes are in place. Westpac is sponsoring part of the recovery programme. The bank’s Avian Domestic Insurance scheme uses the mohua as a "cover bird" and this is contributing $60,000 over three years to developing stoat control methods in the Eglinton Valley. Promoting public awareness of mohua and of the values and ecology of mainland forests is another major aim of the programme. The mohua is one threatened species which is still accessible to the public in mainland forests. Managing the species within these habitats is a priority so that people will be able to see mohua, and other native forest birds, with ease in the future. Stoat control operations are an increasingly important part of other

threatened species recovery programmes in New Zealand. New approaches and attitudes towards the role of predator control in conservation management are now developing. Predator control is now a basic tenet in some threatened species recovery programmes, and controlling both introduced predators and competitors in large "habitat islands" on mainland New Zealand is becoming a reality. As stoat control becomes integrated with other wild animal control initiatives, its cost-effectiveness will improve. The next challenge is the development of techniques for multi-species control programmes so that the full range of pests affecting our forests can be controlled at one time. ® Acknowledgments

CoLiIn O'DONNELL 1s a biologist with the Science and Research Division of the Department of Conservation in Christchurch. He 1s leader of DoC’s Mohua Recovery Programme.