Skewed gender ratios turn bird world into a soap opera

It can be easy to romanticize birds. After all, some 90 percent of bird species are monogamous. And there are sweet stories like male emperor penguins that keep eggs warm and hornbills that sing duets together.

But when adult populations become dominated by one gender, all that sweetness, it seems, hides worlds of wrecked relationships and promiscuity, report András Liker of Sheffield University in England and colleagues in the April 14 Current Biology.

Liker and his team began by compiling data on 187 bird species, examining information on a population’s gender ratio of adults, polygamy, infidelity and rates of divorce — when a pair of birds separates or just doesn’t get back together in the following breeding season. Then the researchers looked for patterns in that data. Monogamy began to unravel when one gender dominated, but there were differences between male-dominated and female-dominated populations.

In female-dominated societies, divorce was twice as common as in species with more males. Divorce can happen in various ways: A paired-up male might desert his mate when she’s been fooling around, or he might leave to pair up with a female with better qualities. Male blue-footed boobies (Sula nebouxii) are known to split up with his mate for those reasons.

Or when there aren’t enough males to go around, an unpaired female might horn in on another relationship, breaking a pair up so that she can get a mate. North Island brown kiwis (Apteryx mantelli) are thought to go that route.

In bird societies that have more of one gender, the rarer sex often takes advantage of the wealth of potential mates to play the field — polygamy becomes more common. This can happen whether it’s the girls or guys with the improved mating opportunities, but it’s a bit more common when males are fewer in number, the researchers report.

When males are greater in number, there’s more infidelity. This could be because females have more opportunity to try out different mates. But it could also have a more sinister source — males that can’t find permanent mates may take to forcing unwilling females.

When Science News’ Susan Milius wrote about bird divorce in 1998, researchers weren’t ready to apply any of their bird knowledge to humans, but it seems that times have changed. “Our results in birds show striking parallels with studies in humans,” Liker and colleagues write. “For instance, divorce rates are higher in both birds and humans in female-biased than in male-biased populations.” They’re still not saying that human and bird populations are alike when it comes to romance, but they note that the bird studies point to some areas of human relationships that might be worthy of more study.