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

James Owen
for National Geographic News
February 14, 2003
 
Oft have I heard both youth and virgin say
Birds choose their
mates, and couples too, this day;
But by their flight I never can
divine,
When I shall couple with my Valentine

—Robert
Herrick, 17th century English poet

The mating game seems so simple for birds. They court, pair up, make a cozy nest, and devote themselves to raising a family—each partner caring for the other as they share the task of parenthood. If only we could be like them.

The English priest and naturalist F.O. Morris, who wrote A History of British Birds in the mid-1800s , had the same message for his parishioners during a Sunday sermon. His perfect role model was the dunnock, a perky little bird of the English countryside.



Morris described it as "unobtrusive, quiet and retiring, without being shy, humble and homely in its deportment and habits, sober and unpretending in its dress." In short, the bird set an example in plain, decent living that his congregation would do well to follow.

Over 130 years later, the dunnock was described in a rather different way by Nick Davies, a British behavioral ecologist.

"A pair of dunnocks had been feeding together, hopping peacefully towards a bush. Reaching it, the male proceeded to one side, the female to the other. Once out of the male's field of vision, the female instantly flew into the nearby undergrowth, where she copulated with a different male dunnock who had been hidden there. Immediately afterward…she rejoined her mate, all the while acting as though nothing had happened."

In recent decades the sex lives of birds have become a lot less worthy and a lot more interesting. In fact, they are proving every bit as complex and messy as our own.

DNA analysis in the 1980s revealed that male partners of many nesting bird pairs often reared chicks that weren't their own. Females, it was shown, are not terribly faithful.

This came as a surprise. Birds are much more likely to pair-bond—form a partnership that extends beyond copulation—than other animals. Social monogamy is thought to exist in 90 percent of bird species. Only five percent of mammals are thought to be socially monogamous, including gibbons, jackals, and tamarins.

"Extramarital Affairs"

For some bird species pair-bonding lasts a lifetime. The Bewick's swan and the waved albatross are two examples. But for others all the "extramarital affairs" clearly take their toll. House martins and greater flamingos are the Elizabeth Taylors of the bird world, with 100 percent of pairings resulting in "divorce."

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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