Swallows keep eggs in several baskets.

Swallows keep eggs in several baskets

Call it nature's version of a baby left on a doorstep. Biologists have discovered that adult cliff-dwelling swallows sometimes carry one of their own eggs into the nest of an unwitting neighboring swallow, and then fly away minus the egg -- a sneaky maneuver called reproductive parasitism that may increase the chance of the egg's survival.

Birdwatchers have long known about another form of reproductive parasitism whereby birds lay their eggs in the temporarily vacant nest of another bird. Some, like various species of swallows, starlings and ducks, keep the eggs within their own species, while cuckoos are known for filling the nests of birds of another feather. But this is the first time scientists have witnessed the transfer of already-laid eggs, report Yale University biologists Charles R. Brown and Mary Bomberger Brown in the Jan. 7 NATURE.

The researchers observed this transfer while studying a community of swallows in Nebraska. They monitored the birds both by sight and by recording the movements of marked eggs between nests. Transferred eggs showed up in 6 percent of the nests. But they add that their estimate of transfer frequency is "undoubtedly an underestimate."

In one instance, a swallow transferred its egg into a nearby nest under the very beak of the nest's occupant. "A fight ensued when the intruder entered with the egg. The intruder was evicted from the nest within 10 seconds, but the egg remained in the nest," according to the researchers. Previous studies have shown that swallows cannot recognize eggs as foreign.

The biological significance of this practice is still unknown. But the researchers suggest it is related to survival. In the general swallow community, roughly a quarter of all eggs fail to hatch. However, only 10 percent of transferred eggs failed, leading the biologists to theorize that when a swallow transfers an egg, it chooses superior incubators as the surrogate parents.

The transfer behavior may have developed in the ancestral swallow nesting grounds of cliffs and canyons, which are the site of frequent rockfalls, say the investigators. Individual swallows may protect against losing an entire clutch to a rockfall by spreading the eggs around.

But in a comment on the report, Cambridge (England) University zoologist N.B. Davies suggests that reproductive parasitism may be more a matter of avian economics. "A more likely explanation perhaps," he says, "is that parasites increase their lifetime reproductive success by reducing the costs of parental care."