For most animals, selecting a mate from a different species is risky business. More often than not, even if the offspring are viable, they cannot themselves reproduce, as in the classic case of mules. But findings described today in the journal Nature reveal that some birds manage to avoid the costs of hybridization. In fact, such interspecies pairing can even be the female's best bet.

Ben Sheldon of the University of Oxford and his colleagues studied hybridization between two closely related species, pied flycatchers and collared flycatchers. Though males of the two species clearly differ in their plumage and songs, female collared flycatchers often pair with male pied flycatchersfar more often than would be expected by chance. At first glance, the mingling might seem fairly disastrous: first-generation female offspring are usually completely sterile. On closer inspection, however, the team found several mechanisms that cancel out the detrimental effects of mixing.

For one thing, if a collared female breeds late in the season, choosing a pied male can actually be advantageous because the "heterospecific" pair will produce more fledglings than a pure collared pair would, owing to interspecies differences in peak performance timing. Second, mixed-species pairs produce more maleswhich suffer fewer effects from hybridization than femalesthus favoring the sturdier sex. Lastly, Sheldon's team found that in a number of cases, although collared females had formed pair bonds with pied males, collared males had actually sired the offspring. Dennis Hasselquist of Lund University suggests in a commentary accompanying the Nature report that perhaps the females cuckold the pied males because they provide better territories. (For their part, the males don't appear to be particularly discriminating. "Males have little mate choice," Hasselquist told Nature Science Update, "if they get a female, they're very happy.")

The new results show that vertebrates may have evolved sophisticated mechanisms to balance out the negative consequence of hybridization, Hasselquist notes. "Such mechanisms might evolve rapidly in a location where two related species overlap," he writes. "Alternatively, it is possible that these mechanisms did not evolve to cope with hybridization, but rather are a side effect of existing female preferences."