When will fairy wrens help other fairy wrens?

Researchers have revealed the nuanced calculations that purple-crowned fairy-wrens make when deciding to defend others from predators.

Purple-crowned fairy-wrens live in social groups, with a dominant breeding pair and up to nine subordinate birds. The subordinates help to raise young birds, and defend the nest against predators.

“Such seemingly altruistic helping behaviour has puzzled biologists for a long time, because it does not make sense to risk your own life to help others without some offsetting benefits, but these have been hard to identify,” says Dr Niki Teunissen, from the Monash University School of Biological Sciences, and lead author on a paper describing the research, published in Current Biology.

Photo of two purple-crowned fairy-wrens, male and female, sitting together in a tree.
A pair of purple-crowned fairy-wrens. Credit: Dr Niki Teunissen

Since 2005, researchers from Monash have been studying a population of around 300 purple-crowned fairy-wrens at the Mornington Wildlife Sanctuary in the Kimberley, Western Australia, which is run by the Australian Wildlife Conservancy.

In this study, they tested the subordinate wrens’ responses to predators by presenting nests with models of different predators: a goshawk, which normally predates on adults; a cuckoo, which targets eggs and some young wrens; and a goanna, which eats young birds and the occasional egg. The goshawk and cuckoo were mounted specimens, borrowed from a museum, while the researchers painted a plastic goanna for the task.

“Our new approach allowed us to work out what incentives drive anti-predator behaviours in social groups,” says Teunissen.

Subordinate wrens defended the nest in a range of ways, including calling for other fairy wrens, physically pushing at the predator and swooping it. Interestingly, the wrens’ responses changed depending on what the predator was, and how likely they were to become a dominant breeding bird in the near future.

“Our findings show that subordinate helper fairy-wrens expecting to inherit the top spot in the group invest in saving the threatened young so that they have a larger group later on when they are a breeder themselves,” says Teunissen.

Two purple-crowned fairy-wrens hiding in a bush
Credit: Dr Niki Teunissen

“In other words, they are actually saving their own future helpers.”

Against the hawk, however, the birds were willing to protect relatives or potential mates, but they did not come to rescue competitors.

“We’re trying to explain an extremely risky behaviour,” says Professor Anne Peters, senior author on the paper. “Even a cuckoo that isn’t directly a threat as a predator gives you a big peck if you harass it too much, and you can get hurt. And a hawk of course is extremely dangerous – you could get killed.

“This sort of behaviour could have really, really big costs. So we were expecting also big benefits, and those benefits aren’t too obvious. What our study shows is that you really have to look very closely, and understand your study species extremely well in order to be able to explain it.”

She says it shows that the birds aren’t being truly altruistic: instead, they’re acting in ways that preserve their chance of breeding.

“This notion of altruism is slowly diluting away as we understand better why these helpers are doing what they’re doing.”

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