Female ducks can double eggs by trickery. (Science News of the week).

Female goldeneye ducks can make cheating pay big-time, doubling the number of their offspring by sneaking off to lay eggs in nests other than their own.

Bird-watchers have long known that many female waterfowl slip extra eggs into nests of other birds, notes Matti Ahlund of Goteborg University in Sweden. However, he says, the goldeneye ducks have surprised researchers with the success that some of the females achieve by this kind of cheating.

Sneaky male birds of other species have doubled their offspring by mating with females other than their nesting partners. In the goldeneye ducks, "we describe a female parallel," Ahlund and his Goteborg colleague Malte Andersson say in the Dec. 6 NATURE.

The female ducks don't mate with males other than their regular partners, says Ahlund. Rather, they sneak into a tree cavity where another goldeneye female, off on some errand, had started laying a clutch of eggs. The intruder lays an egg, which the nest owner then raises. Success with this tactic could create a considerable advantage for clandestine layers and could help explain how the practice evolved, the researchers say.

Goldeneye ducks have settled widely across Europe and North America. Ahlund and Andersson monitored the fortunes of pairs nesting at Lake Mjorn in southwestern Sweden. During one year of their study, they observed 36 nests and analyzed egg proteins to identify the respective mothers for 383 duck eggs.

Females took one of three approaches to laying eggs. Some laid eggs only in their own nests; others, only in foster nests. However, neither of those groups produced as many fledglings per mom as did females that started the season by laying eggs in other ducks' nests and then raised a clutch of their own. One champion farmed out 14 eggs before laying 7 in a nest of her own.

Biologists have noted some 200 bird species slipping eggs into other birds' nests, says Bruce Lyon of the University of California, Santa Cruz. Just what drove the practice's evolution remains a topic of debate. One scenario suggests that disadvantaged birds, perhaps ones too lowly to win and hold onto a nest site, get an advantage from stealth laying because it beats laying no eggs.

Another scenario centers on the boost in reproductive success that comes with the stealth-egg-laying technique. Researchers have documented such boosts before, but never ones that resulted in a doubling of offspring, as claimed in the new study, Lyon says.