The advantages of infidelity

Although not all birds mate for life, many do, and often mated pairs will stay together at least for the duration of a reproductive season.  Birds are sneaky, however, and some “sneaker” males will often try to stealthily mate with females within pairs.  Behavioral ecologists have many theories about why females engage in these extra-pair copulations. Since the birds can store sperm in specialized internal pouches, it may act as insurance against the possibility that their mate is infertile. Or it might simply be easier not to resist advances from extra-pair males.
The fact that extra-pair offspring are often superior to their mated-pair half-siblings — enjoying higher survivorship, greater performance abilities and faster growth rates — has led researchers to speculate that females copulate with extra-pair males because they are genetically superior to their mated males, and thus pass these good genes on to their offspring.  However, a new study in the journal Current Biology suggests that superiority of extra-pair offspring may instead be based on the environment.

Michael Magrath of the University of Groningen in the Netherlands and his coauthors found that in an experiment with blue tits, extra-pair offspring were laid and hatched earlier than mated-pair offspring, with 75 percent of extra-pair chicks being produced in the first half of the female’s clutch. Said Magrath in a statement:

“Generally, earlier hatching chicks perform better than their later hatching siblings because they gain an initial size advantage, giving them the edge in competition for food during the nestling period.”

When they corrected for laying date, the researchers found no differences in quality among the hatchlings, suggesting that the effect was not genetic, but instead based on the environment.

Why the offspring sired by extra-pair males hatch earlier, however, remains a mystery, and fodder for future study.

Magrath, M., Vedder, O., van der Velde, M., & Komdeur, J. (2009). Maternal Effects Contribute to the Superior Performance of Extra-Pair Offspring Current Biology DOI: 10.1016/j.cub.2009.03.068