If you think I’m having your babies… (Image: Wim Weenink/Minden Pictures/FLPA)

AS THE song would have it, birds, bees and even educated fleas fall in love. For some birds, however, the love turns sour – they get divorced.

Many birds may seem like model parents, with males and females investing more equally in caring for their young than the average mammal. While 85 per cent of bird species are socially monogamous – they form pair bonds and share the workload – divorce is common, occurring in 92 per cent of these species, including the humble great tit (pictured).

By analysing data recorded from across 64 species, Antica Culina at the University of Oxford and colleagues found that birds were most likely to divorce when their breeding success was low (Biological Reviews, doi.org/wnw).


Culina found the signs of a doomed partnership emerge early – pairs in which the female produced a low number of eggs or laid them relatively late were less likely to stick together for a second breeding season. It is possible that unimpressed males may decide they can do better. But females are believed to have some control over the number of eggs they lay, so a small clutch size may actually be a sign of female discontent, says Culina. “It might be that she’s already made the decision, and because she doesn’t like him very much, she won’t make many eggs.”

“Divorce can be beneficial or detrimental for an individual,” says Tamás Székely of the University of Bath, UK. “By ditching a poorly performing mate, a bird may well hope to find a better one.” But he says divorce can have disastrous consequences, especially if there aren’t many other males or females to choose from.

Culina’s study found that, when birds do break up, it’s the females that benefit. “Females who divorce gain better breeding success with a new partner, but males who divorce show no improvement,” she says. This might indicate that it is the females that decide it’s time to find a new mate. A male wanting to leave a partner risks leaving territory, too – so they may have more to lose.

How does a bird decide whether to stay or go? Do they monitor each other’s behaviour, for example? Not much is known, says Székely.

This article appeared in print under the headline “Why bird break-ups are bad news for the boys”