June 8, 2001

When Mr. Wrong Is Mr. Right

By LILA GUTERMAN

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

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 at the University of Oxford, led a study that set out to answer the question "Why would a female