Sex Discrimination Starts in the Nest

Most parents may try to treat their kids equally, but evolutionary theory suggests that animals may benefit by favoring offspring of one sex over the other. That's because males and females may require different amounts of energy to raise, or because one sex may be a better bet for passing on genes. Now, Dutch researchers have documented that one bird species gets a head start on sexual discrimination: In today's issue of Nature, they describe a tropical warbler that chooses the sex of its offspring before they're even hatched.

Jan Komdeur, a biologist at the University of Groningen, the Netherlands, and his colleagues have been studying more than 300 warblers on Cousin Island in the Seychelles. Just as many kids today remain at home long after they could set out on their own, the female offspring of the Seychelles warbler tend to hang around the nest for a few years, waiting for a chance to breed. They help their parents by taking turns incubating the lone egg and by bringing food to the nest. If a breeding territory is crawling with nourishing insects, helpers ease parents' workload and give them a better chance of successfully raising the next brood, explains Komdeur. But if food is scarce, the helpers become a drain, and male chicks, which tend to leave the home turf earlier, are a better bet from the parents' point of view.

But could the warblers actually order up chicks of the preferred sex? To test that idea, Komdeur used DNA analysis of blood samples to check the sex of the identical-looking nestlings during the 1993 to 1995 breeding seasons. Sure enough, parents nesting in insect-poor habitat averaged 77% sons, while breeding pairs living on territories with abundant food produced 87% daughters. What's more, the parents can switch strategies. When Komdeur moved the breeding birds from poor to rich habitats, they began to have females rather than males, and vice versa.

Other researchers are hailing the finding. Ecologist Patricia Adair Gowaty at the University of Georgia predicts the paper will become an "instant classic," calling it "the first time that [changes in sex ratio in birds] have been shown so convincingly." But just how the birds manage to change that ratio remains a mystery: Sex in birds is determined by which chromosome the egg carries, and each egg starts out with a 50-50 chance of being male or female. So far, the warblers haven't revealed their secret.