Divorce is widespread in monogamous birds, and now researchers show for the first time that the consequences are beneficial. They’re trading up! Especially the females; males tend to have more to lose. The findings were published in Biological Review last month.
More than 85 percent of birds are socially monogamous. And only a quarter of these are also genetically monogamous. In the rest, different genetic mating patterns pop up when birds mate outside their breeding pair (it’s called extra-pair copulation) or when they change partners between breeding seasons because of widowing or divorce. It’s considered a divorce in birds when both individuals in a pair are still alive but at least one mates with a new partner in the next breeding season.
While common, it’s been unclear if divorce is adaptive and if it improves the quality of a mating decision -- or maybe it’s just a product of circumstances? After all, having both parents around to care for eggs and chicks is essential in the majority of these monogamous birds.
To see if divorce is an adaptive behavioral strategy, a University of Oxford trio led by Antica Culina analyzed datasets from 81 studies on breeding success across 64 species of socially monogamous birds. From owls to ducks to songbirds (like the great tit above), divorce is triggered by relative low breeding success, they found, which is changed for the better after the split.
Furthermore, earlier stages of breeding are better predictors of divorce than later stages. Pairs where the female produced a low number of eggs or laid them relatively late were less likely to stick together for a second breeding season, New Scientist reports; maybe an unimpressed male thought he could do better. However, females may control the number of eggs they lay, so a small clutch size could be a sign of female discontent. "It might be that she's already made the decision,” Culina tells New Scientist, “and because she doesn't like him very much, she won't make many eggs.”
So who called it off? Turns out, females benefited from divorce more than males. They enjoyed an increase in breeding success with a new partner between successive breeding attempts -- with different stages of the breeding cycle improving at different rates. Males who divorce show no improvement, Culina adds, and they also risk losing their territory in the process.