When Mr. Wrong Is Mr. Right

For some flycatchers, mating within the species is for the birds

By LILA GUTERMAN

Mating across species boundaries sounds downright unnatural. Such peculiar pairings often don't produce any offspring at all, or they give rise to sterile progeny, such as mules. But for some birds, mating with another species may produce distinct evolutionary payoffs.

Ben C. Sheldon, an evolutionary biologist...