"Love" Birds: Mated for Life but Bound to Cheat?

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This is the term used by ornithologists to describe liaisons that break up between or during breeding seasons. The reasons why there are species that split up when others stick together have been the subject of much study in recent years. And some researchers believe their findings could help us understand why many human relationships also end in tears.

Take the Kentish plover (also known as the snowy plover), a bird that suffers from high divorce rates. Decisions that influence this behavior were recently investigated by the School of Biological Sciences at the University of Bristol in Britain.

"Basically, it's a case of the benefits of divorce outweighing the costs," said Innes Cuthill, a zoologist who led the research team. "For some reason there's a male-biased sex ratio among Kentish plovers. This means there are plenty of opportunities for the females to stray. The benefit of abandoning one mate for another is having more babies—provided the abandoned parent can bring up the kids alone. The male plover is usually able to do this."

Yet not all males appear willing to embrace life as single parents. Observations suggest they try a cunning psychological ploy to prevent their partner fleeing the nest.

Cuthill explains: "A parent in poor physical condition is less likely to be able to bring up the kids alone, so there's less advantage in the female leaving them. Instead they stay and help. We think males may be losing weight deliberately to keep their partners.

"Isn't this a classic male tactic? When you're asked to wash up the dishes by your wife, you smash a few—whereby demonstrating you're incapable of doing it yourself," Cuthill said.

Other birds have greater reason to stay together. With species like the great skua the costs of divorce ensure most pairs remain faithful.

Great Skuas

Robert Furness, professor of biology at the University of Glasgow in Scotland, has studied divorce among great skua colonies.

"Unlike Kentish plovers, both parents must contribute if the chicks are to survive. The male skua does almost all the foraging and feeds his partner, while the female stays on their territory and guards the nest," Furness said.

Yet there are occasions when divorce is the best option. "Forming a new pair-bond takes a long time and usually delays breeding," Furness said. "Birds that divorce tend to be less successful than the average pair. But they may be more successful than if they stayed with a partner they weren't compatible with."

So if a male bird isn't pulling his weight as a food provider, or is less fertile than his neighbors, the female may well take her chances elsewhere.

Most of us can see where the female skua is coming from here, even if we think her a little hardhearted, but could such behavior help us understand human motives for divorce?

Cuthill thinks so. "Obviously individual motivations in humans and birds are going to be different, but the general evolutionary reasons for divorce are probably similar.

"It's dangerous ground, so in a sense we are throwing this open to braver people. It's an interesting proposition for a mathematician or an economist because the best thing to do depends precisely on what everyone one else is doing. For example, if everyone's being faithful there's little point in leaving your partner as there are no potential mates.

"Humans, like different bird species, have probably evolved to have a divorce rate that is roughly appropriate for the conditions we live in. And it's likely to be something that's adaptive, as it is in birds," Furness said.

Whatever the case, remember the little dunnock. If your valentine disappears behind a hedge tonight, or receives a card you didn't send, beware. A secret admirer could be waiting in the wings.

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