The female bird has a unique ability to influence which eggs hatch first. She can store sperm for days in her reproductive tracts, releasing some from the outsider to fertilize some of her eggs, giving them a few hours head start, and a few hours later release some from her mate for the remaining eggs, ensuring that he will hang around to help raise the young.
It's the best of both worlds.
Scientists who have been aware of the infidelity among birds have generally thought that the males were the real cheaters, possibly because most of the scientists were men. But some research indicates it's really the females that are most eager to find alternative sources of DNA.
Biologist Bridget Stutchbury, who has authored two books on the subject, said the female is the most opportunistic, and she doesn't have to look far to find a male who's willing to fill her needs, at least as far as the birds in her Canadian backyard are concerned.
"When the female sneaks around, she goes next door, and then visits the male, mates with him, then comes back," she said during an interview on Canadian television. "And her own mate does all the work raising the young, but he's not always the father."
Other research underscores the importance of the male's role in taking care of the nest, and it offers a strong warning to the females: When the hanky panky is over, preen a little.
Scientists with the Konrad Lorenz Institute of Ethology in Great Britain captured female blue tits just after their eggs hatched. These birds do not appear to be all that colorful to human eyes, but the feathers on the tops of their heads reflect ultra-violet light, which birds can see, giving them more color and the necessary sex appeal.
The researchers smeared the tops of their heads with either "duck preen gland oil containing UV-blocking chemicals or the oil alone." That had no effect on the females, but males whose mates had been coated with the oil containing UV-blocking chemicals made fewer hunting trips to feed their brood. So beauty still matters.
Some birds don't even try to help raising their young. The notorious cuckoo likes to lay its eggs in another bird's nest, and then go on about its life. If the cuckoo's egg hatches first, the chick will nudge any other eggs over the side of the nest, thus becoming an only child. It's nasty business.
But purple martins and swallows in Europe have figured out how to beat the cuckoo. They build their nests near humans, and cuckoos don't care much for our species, thus leaving the nests alone.