At first glance, the "sexy son hypothesis" makes perfect sense. According to this pillar of evolutionary biology, a female who chooses a high-quality male will have sons who inherit dad's allure. They, too, will therefore have their pick of females, allowing mom to hit the jackpot: grandmotherhood.

But when scientists followed male flycatchers whose dads were real catches (as judged by a forehead patch that is this bird's equivalent of perfect abs), they found no such thing.

The...