It's quite common for a female song sparrow to stray from her breeding partner and mate with the male next door, but a new study shows that sleeping around can be costly.

The study found that offspring conceived outside sparrows' social pairs go on to have lower reproductive success than within-pair offspring. The findings throw a monkey wrench into theories about why ostensibly monogamous animals might be inclined to cheat.

"These results are remarkable because they are completely opposite to expectation," study researcher Jane Reid, of the University of Aberdeen, said in a statement. "They show that females suffer a cost of promiscuity because they produce worse offspring through extra pair mating. Rather than answering the question of why females should mate promiscuously, [these results] have blown the question wide open."

Most bird species display some form of monogamy. Bonded pairs stay together for a breeding season, a few seasons, or sometimes for life. But beneath this veneer of monogamy, there's plenty of hanky-panky in most species. Why this promiscuity exists in monogamous species is "one of the biggest remaining enigmas in evolutionary ecology," Reid said.

One hypothesis for this is that when a female strays she makes it count by mating with a male of higher genetic quality than her social mate. The result is higher-quality offspring that have a better chance of carrying a female's genes into future generations. This study, however, turns that explanation on its head.

The researchers studied a population of song sparrows in Mandarte Island in British Columbia, Canada for nearly 20 years. Each year starting in 1993 the team drew small blood samples from nearly every hatchling in the population and used genetic markers to see who fathered each bird. They found that 28 percent of all chicks were fathered by males other than a female's socially paired mate. Thirty-three percent of broods had chicks that were fathered by multiple males.

The researchers tracked both within- and extra-pair offspring throughout their lives. They found that extra-pair offspring had 40 percent fewer offspring of their own, and 30 percent fewer grandoffspring, compared to within-pair offspring.

The study was published in the June 2012 issue of the journal American Naturalist.