Sex-allocation theory predicts how the sex ratio of offspring should vary with the mother's physical condition. Applying this theory has helped in retrieving a charismatic parrot from the edge of extinction.
The kakapo, a nocturnal, flightless parrot found in New Zealand, is an endangered species, its rarity and eccentricity making it a high-profile test for conservationists. At a meeting last monthFootnote 1, Mick Clout (Univ. Auckland) reported a scientific success story in reducing the bird's risk of extinction. Providing extra food to improve fledging success seems to have inadvertently resulted in breeding females producing an excess of male young, as predicted by evolutionary theory1,2. Given this finding, conservation workers decided to withhold extra food until after the females had laid their eggs, resulting in a much-needed boost in the number of female fledglings this year.
The kakapo (Strigops habroptilus; Fig. 1) was once common in the three largest islands of New Zealand, but its distribution shrank following the spread of black rats and stoats introduced from Europe in the 1870s and 1880s. By the 1950s, kakapo were extinct on North Island and only 18 remained in a remote and mountainous part of South Island. The stoats are thought to have eaten not only eggs and chicks, but also incubating females, so all 18 survivors were male. The only remaining wild females were on Stewart Island (south of South Island), which stoats had not colonized, although rats and feral cats were present. A bold decision, taken in 1982, was to capture all of the remaining birds and release them on predator-free islands, including Codfish Island, off the coast of Stewart Island. A world population of 62 birds in 2001, with only 21 adult females, created an urgent need for more females, yet only six female fledglings had been produced since 1982.
In most years, kakapo feed mainly on foliage and other poor-quality foods that are insufficient to allow breeding. Egg-laying is restricted to those years in which certain trees — podocarps — undergo mass fruiting, known as masting. Since 1989, supplementary food, such as nuts and sweet potato, has been given to some individual kakapo from dispensers in their home ranges, resulting in a 15% increase in body weight for fed birds. Unlike natural masting, supplementary food did not seem to induce breeding. But fed females spent less time off the nest, which increased chick survival.
The theory of sex allocation proposed by Trivers and Willard1 states that females in better physical condition should produce more offspring of the sex that shows the greater benefit from the improved condition. Kakapo reproduce in leks — communal display grounds in which males fight to defend small territories within which they display vigorously. Females visit just to select a mate before raising the young alone elsewhere. Studies of various lek-breeding species show strong competition between males, with a few high-quality individuals obtaining almost all the matings3.
Male kakapo fight, sometimes to the death4, and larger males are more likely to mate. Male kakapo also grow faster and larger than females, and are thus presumably more costly to raise5. So if Trivers and Willard's hypothesis applies, mothers in good condition should produce more sons. Females provided with supplementary food indeed produced significantly more sons (an average of 67% against 29% for birds not given supplementary food)2. There is accumulating evidence that maternal condition regularly affects the offspring sex ratio in a range of species6, including tree swallows7, lesser black-backed gulls8, great reed warblers9 and another species of parrot10.
A profuse fruiting of podocarp trees was expected to trigger kakapo breeding on Codfish Island this year, so the New Zealand Department of Conservation concentrated all remaining 21 adult females there. Supplementary feeding was delayed until after egg production. The idea was to avoid boosting female condition too much before laying, to avoid a male bias in offspring. Extra food was still provided after laying to maximize fledging success.
The strategy seems to have worked: as Clout described at the meeting, 15 of the 24 young fledged this year were female. This is a neat example of how behavioural ecology can benefit conservation, although the mechanism by which females manipulate the sex ratio remains a mystery.
A common problem in conservation is that endangered species are pushed into suboptimal areas as a result of predation by introduced species or exploitation in their core range. Among the bird species that have only really flourished once they were moved are the nene in Hawaii, the Lord Howe woodhen on Lord Howe Island, the takahe in New Zealand and the red kite in Britain. Intensive manipulation of the kakapo has had its successes, but the long-term solution will surely lie in finding ways of restoring predator-free areas on the main islands of New Zealand.
*International Ornithological Congress, Beijing, China, 11–17 August 2002.
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