Most songbirds form lasting pair bonds, raising chicks with a partner. But this doesn't always stop them from getting something on the side – in some species rather often. Ornithologists have noticed that when female birds “cheat” on their partner, the resulting offspring usually have older fathers. A new study narrows the options on why male birds get sexier with age.

We'd expect older males to be, if anything, less fertile than younger ones as they have less energy for extramarital shenanigans, so the findings pose a puzzle. Nevertheless, the phenomenon has been confirmed across many species. 

Dr Antje Girndt of Imperial College London and the Max Planck Institute for Ornithology decided to test two possible explanations. The “male manipulation” hypothesis proposes older birds are initiating sex with partnered female birds more often, perhaps even coercing them. The alternative, referred to as “female choice”, proposes partnered birds like to sneak off with the avian equivalent of a silver fox.

To decide between the theories, Girndt and colleagues had to spend months watching captive house sparrows during mating season, testing the DNA of offspring to confirm the father, which in 10 percent of cases was not the female's partner. A paper in Scientific Reports rules out the male manipulation theory, at least in sparrows. There were no signs of older males making additional attempts to lure females away from the marital nest, nor being more aggressive when they did so.

However, the female choice theory didn't stack up all that well either. Although most successful matings were initiated by the females, there was no evidence they were more likely to approach older males.

Having rejected the two theories she was trying to test, Girdnt fell back on the possibility that older sperm is more likely to fertilize eggs. “We found that there is likely to be a biological effect, rather than a behavioral one,” co-author Dr Julia Schroeder said in a statement. There may be something intrinsically better about older males' sperm, but Girndt notes female sparrows share a trait with many kangaroos and sharks – they can store sperm for extended periods.

Consequently, it is possible the females are in some way controlling which sperm fertilizes their eggs, storing it up from their partner and both older and younger paramours. In this scenario, which the team describes as hard to investigate, the females are enjoying their fun with birds of all ages, but choosing to be fertilized by the older males, perhaps because a bird that has survived so long must have good genes.

It's inevitable that people will anthropomorphize such work, and it might not have been done were it not for possible human implications. Nevertheless, while cheating is common in humans, there is increasing evidence that having children through an extra-marital affair is actually quite rare.