Biologists have long known that when it comes to hatching eggs and raising the young, females in certain species often try to cheat, sneaking their eggs into the nests of other brooding females. This phenomenon, known as conspecific brood parasitism (CBP), is particularly widespread among ducks. Why the host female accepts eggs and raises chicks that are not her own, however, has remained enigmatic. Perhaps, it was reasoned, CBP might be explained through kin selection. That is, if the host is related to the transplanted chick, or parasite, she can enhance her own reproductive fitness by promoting the survival of her relatives. Now, more than 15 years after G¿teborg University zoologist Malte Andersson proposed that idea, he and his colleague Matti ¿hlund have provided evidence to support it in a report published this week in the Proceedings of the National Academy of Sciences.

The researchers examined relatedness among female goldeneye ducks, using a novel technique of so-called protein fingerprinting. From each egg in a clutch, they sampled a small amount of the egg protein albumin (without affecting the hatching success of the egg), which is maternally derived, and generated a pattern of bands, or fingerprints, unique to that chick. Andersson and ¿hlund were then able to look at patterns of relatedness by comparing the various fingerprints. Statistical analyses showed that hosts and parasites were in fact often related. Whether the host and parasite recognize each other as kin is not entirely clear. It may be that because the females tend to return to their birth sites year after year to nest, parasites might be depositing their eggs in the nests of relatives just by chance. But the researchers suggest that a mechanism for kin recognition is likely.

Aside from CBP, waterfowl display a wide range of breeding systems, including solitary nesting and communal nesting. According to Bruce E. Lyon and John McA. Eadie, who wrote a commentary accompanying the PNAS report, "The challenge now will be to obtain the necessary data on relatedness, ecological constraints and the costs and benefits of these many alternatives. Conspecific brood parasitism¿the strange phenomenon noted by waterfowl biologists half a century ago¿may well turn out to be a fascinating and important link in the evolution of avian breeding systems."

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