ScienceWise - Winter 2011

Why do birds babysit?

Explaining apparent altruism in the purple-crowned fairy wren

Australia’s separation from the other major land masses of the world has led to the evolution many extraordinary species of plants and animals. One such animal is the purple-crowned fairy wren (Malurus coronatus) which is only found in certain regions of Northern Australia. One peculiar aspect of the behaviour of this tiny bird is that adult birds will frequently help in the raising of offspring that are not their own.

In a world dictated by the passage of favourable genes from one generation to the next, this seemingly selfless behaviour has puzzled biologists for some time. However, recent research at the Australian National University may be set to shed new light on this phenomenon.

Dr Michelle Hall, a visiting fellow in the ANU Research School of Biology, was recently part of a long-term study on the cooperative breeding habits of purple-crowned fairy wrens. The study was led by Sjouke Kingma from the Max Planck Institute for Ornithology with Dr Anne Peters from Monash University as senior author.

Dr Hall says “Fairy wrens are habitual cooperative breeders, with breeding pairs receiving help in the nest from non-breeding helpers. But what our study has shown for the first time is that essentially this helping in the nest boils down to both family and inheritance.”

With individual birds looking very much like each other (to a researcher anyway!), the only way to keep track of who’s who was to capture each bird using mist nets then tag their legs with coloured bands. “The great thing about the banding technique is that you can read them from quite some distance” Dr Hall explains, “So unlike number tags, it wasn’t necessary to keep recapturing them to identify each bird, which would of course, disturb their natural behaviour.”
The study was quite extensive, tagging and following every member of the Fairy-Wren population along a 10km stretch of river. “We spent many hours watching nests, monitoring who was feeding the nestlings, keeping tabs on changes in social groups, and recording when and where youngsters obtained breeding territories.” Dr Hall says.

“By looking at the dynamics of these social groups we realised that when it comes to helping raise the young, it is a matter of relatedness. For example in purple-crowned fairy-wrens over 60 per cent of helpers live with both parents in the territory they hatched on and over 90 per cent live with at least one parent. So the young they help raise are usually siblings or half-siblings.”

“We also found that in general helpers that are more closely related to the nestlings provide more help and their efforts thus see more of their shared genes entering the gene pool. In other words, helpers adjust their behaviour to maximise these indirect benefits by helping kin.”

Such apparently selfless cooperative breeding behaviour has intrigued biologists for decades. However, this new research suggests that the birds performing the helping in this case, have very strong genetic ties to the offspring being raised which makes perfect sense from an evolutionary perspective. Cooperative insects such as bees relinquish their personal ability to breed entirely in favour of offspring from a single queen, though of course all the bees that are produced carry genes closely related to those of the workers who raise them.

While the findings present “an elegant theoretical solution” to the problem of unexpected animal altruism, Dr Hall said that they are by no means universally observed.

In a twist, some helpers in purple-crowned fairy-wrens, and other cooperative breeders, also provide help for unrelated young in the nest. But it turns out these helpers are also playing a selfish game and maximising direct benefits by producing future helpers of their own. They help when their chances of inheriting the current breeding territory are greater, and thus are helping to raise their own future assistants.

It would seem that even in the world of the purple fairy wren, there’s no such thing as a free lunch!
The study’s findings have been published in the latest edition of The American Naturalist.

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