Dieting to save a species: mother parrots that eat less avoid excess of sons.

Now that conservationists are counting calories for the endangered, flightless parrots of New Zealand, the birds are recovering from a shortage of female chicks, biologists report.

The world population of the kakapo (Strigops habroptilus), a hefty, nocturnal parrot, numbers only about 86 birds, says Bruce Robertson of the University of Canterbury in Christchurch, New Zealand. About 5 years ago, conservationists realized that among the birds that they were tending, only 30 percent of the offspring were female. That's hardly the way to make a lot of new kakapos.

Using a bit of evolutionary theory called sex allocation, researchers proposed that feeding the females less could shift the male-female ratio of chicks. Now, a genetic analysis of chicks from the 2002 season shows that the scheme works, Robertson and his colleagues report in an upcoming Biology Letters.

Kakapos once waddled all over New Zealand, but European settlers and their predatory animals found the ground-dwelling, strong-scented birds easy to catch. In the 1980s, conservationists whisked the last 51 known kakapos to island sanctuaries.

The birds rummage along the ground for fern rhizomes and plants from which they suck juices. Every few years, rimu trees burst out with a bumper crop of their tiny orange fruits, and the kakapos feast and lay eggs.

To boost reproduction, conservationists had provided frequent feasts of apples and other treats. Females did plump up, and more chicks tended to survive.

However, the improvement in female condition might have backfired, biologists including Robertson and Jose Tella of Donana Biological Station in Seville, Spain, suggested at the beginning of the decade. A clue came from the mating system, in which kakapo males set up displays and females review them.

Kakapo males spend summer nights meticulously clearing dirt patches where they then spend hours calling in females. Success in attracting a mate varies greatly from one male to the next.

Sex-allocation theory predicts that in such species, females will produce an abundance of sons when moms are fit and their sons are likely to grow up capable of attracting mates. During hard times, puny sons generally get shut out of fatherhood, so extra daughters are a better bet.

In 2001, wildlife managers put the heaviest females on a restricted diet but continued to feed the thin ones liberally. Robertson and his colleagues now report that the diet indeed ended the excess of sons. The females on short rations laid 9 male and 10 female eggs, and the already lean females produced 7 male eggs and 9 female ones.

"It's a nice application of evolutionary theory to conservation biology," says Timothy Wright of New Mexico State University in Las Cruces, who has also studied parrot conservation.