"Married" and "settled down" are two different concepts for some birds. Female collared flycatchers settle their nests with one mate but often lay eggs fathered by a studlier male. Now, researchers maintain that females are in charge of these extra-nest dalliances and time them for when they're most fertile.
To the casual observer, most songbirds appear monogamous; DNA testing, however, reveals that anywhere from 10% to 75% of females cheat on their nestmates, depending on the species. Researchers studying the collared flycatcher know that about 40% of females have affairs, typically with males sporting a larger-than-average white spot on their foreheads. These males tend to father plumper offspring. Although previous work suggested that the female picks her paramours, researchers couldn't tell how she arranged the trysts to get the most reproductive bang for her, well, bang.
Rather than trying to catch females in the act, a team led by behavioral ecologist Ben Sheldon of the University of Oxford, U.K., let the eggs tell the story. Females can store sperm from a mating and then fertilize successive eggs. After an egg is laid, researchers can tell how much sperm the female has left by counting the number of sperm caught between the egg membrane and the shell. The researchers designed an avian prophylactic that would prevent partnered he-flycatchers from fertilizing any eggs. By counting the number of sperm in six consecutively laid eggs of females with blocked partners, Sheldon's group could detect if she had cuckolded him.
In 9 of 15 pairs, the number of sperm in each successive egg declined, indicating that these females stayed with their beaus, the team reports in the 16 April Proceedings of the National Academy of Sciences. In the other 6, however, the sperm count spiked in at least one egg, showing that the females had received sperm from males other than their nestmates. Females paired with a male with a large forehead patch didn't cheat, whereas most of the females whose male had a small patch cheated at least once--and did so in the middle of their fertile period. Together, these results suggest that the females run the rendezvous, Sheldon says.
"It's a really cool paper," says behavioral ecologist David Westneat of the University of Kentucky, Lexington. But he cautions that the study's lack of behavioral data means there's still room for interpretation on who really chooses the extra-nesting liaisons.