When female sparrows cheat, males make the kids suffer

A male sparrow on the island of Lundy that was included in the study. Photo / A. Sanchez-Tojar

Talk about keeping your eyes on the sparrow. That age old advice to humans to stay focused is not lost on male sparrows that live on an island off the coast of England.

According to a new study, they keep a sharp eye on what it calls cheating "wives".

As pretty birds naturally hop from nest to nest in search of genetic diversity for the eggs they lay, their partners take note.

"The males may use cues from the female's behaviour during her fertile period - for example, how long she spends away from the nest," said Julia Schroeder, the lead author.

Once the eggs hatch, the males get passive-aggressive, providing less food for the brood when they sense the babies they're caring for probably aren't their own.

Schroeder's team of scientists from Australia and Germany came upon this finding while trying to figure out why female sparrows are unfaithful. Was it to strengthen offspring? Avoid inbreeding? Improve fertilisation?

"To further our understanding of the evolution of female polygamy," the authors wrote in the study, "it is crucial to understand not only the benefits but also the costs of females producing EPO, Erythropoietin, a hormone that controls red blood cell production."

It's a popular theory among scientists that as a trade-off for being a cuckold to a philandering female, males "should provide less care to a brood that contains more EPO than to a brood of the same female that contains no or fewer EPO." The study was published in the American Naturalist.

For the research, the scientists studied sparrows on the island of Lundy in the Bristol Channel for a dozen years. Schroeder, a researcher in the Department of Life Sciences at Imperial College London, could not be reached when the study was released.

But in a statement, she said males in the study "changed their behaviour based on their partner. When they switched from a faithful partner to one prone to infidelity, they provided less food for their brood. Females might also change their behaviour when paired with a less lazy male, cheating less with a more attentive father."

Cheating less, but not stopping altogether. Before drawing any conclusions, keep in mind that male sparrows on the island couldn't exactly point feathers. They're hypocrites that constantly cheated themselves.

"Biologists believe that the male birds are unfaithful to ensure that they father as many chicks as they can, while females are unfaithful with males of better 'genetic quality' - ones that are fitter and could produce stronger offspring," a statement announcing the study said.

Two hundred males and nearly that many females were observed for the study. Over the 12 years, they formed more than 300 unique pairs and hatched more than 850 broods. There were some of what the researchers called "sparrow divorces," a topic perhaps deserving more study, "but most changes in life partners were due to death," they said.

Schroeder's team of six genotyped each sparrow and traced their family trees, paying close attention to unfaithful partners of both genders. The researchers said they chose Lundy because few birds leave the island or migrate from the mainland.

Males had no way of knowing they were being deceived by their partner through sensory perception such as touch or smell. But it was important to keep their eye on the female sparrow, Schroeder said.

The more faithful the female, the more diligent the male. "If chicks were switched into a nest where the female was faithful, then the father at that nest kept up his hard work providing for the chicks, suggesting they have no mechanism, such as smell, to determine which chicks are theirs," Schroeder said.