Fairy-wrens babysit if they’ve got a good chance of mating with the parents

Why do animals help each other? Altruism in nature is a puzzle with many solutions, and fairy-wrens have found a few.

A team of researchers based at Monash University has found more detail on why purple-crowned fairy-wrens will help raise offspring that don’t belong to them. It seems that being related to the offspring, or lining up a potential mate, are both good reasons for babysitting.

The latest research is published in Royal Society Open Science.

“While our results are certainly novel for the field, they did not entirely come as a surprise,” lead author Dr Niki Teunissen, an adjunct research associate at Monash, tells Cosmos.

“In previous research on these purple-crowned fairy-wrens I have shown that non-breeding fairy-wrens – ‘helpers’ – benefit from sharing a group with particular group members.

“More specifically, they form close social bonds with group members that are relatives and potential mates as evidenced by frequent affiliation such as allopreening, where birds sit closely side by side and clean each other’s feathers.

“I have also shown that when there’s a dangerous predator nearby that could do harm to group members, helper fairy-wrens are selective with who they will help defend.”

Named for the males’ brilliant violet heads, purple-crowned fairy-wrens are native to northern Australia. They live in social groups, with one breeding pair and, usually, subordinate wrens that can help with the breeding pair’s offspring.

Purple-crowned fairy-wren with purple head and blue tail.
A male purple-crowned fairy-wren with food. Credit: © Niki Teunissen/AWC

Teunissen and colleagues have been observing the birds for years in the Kimberley region of Western Australia, at a site 8 hours’ drive from Broome.

“We really are in the middle of nowhere, following these birds during the wet season, as this is when most breeding commonly takes place, and we are completely cut off from the outside world due to regular flooding of access roads,” says Teunissen.

“These wrens live along creeklines, so observing them means fighting our way through very dense vegetation, and frequently encountering snakes, spiders, and whatever other animals and insects you can think of.

“The environment is pretty unforgiving – it is very hot and sweaty and we frequently have to cross flooded creeks to get to where the birds and their nests are. Pandanus – the vegetation these wrens prefer – also has lots of nasty thorns which dig their way into our skin, causing frequent irritation and rashes.

“It’s certainly no easy feat, but it’s all worth it when, after watching a female fairy-wren for a while you finally see her picking up nesting material and leading you to her secretive nest location.”

The researchers attached tiny coloured metal bracelets to the birds so that they can identify them and record each wren’s behaviour.

“This might sound pretty straightforward, but it actually requires quite a lot of practice to be able to tell the colours apart in dim light and from a fair distance, and to be able to see all the different colours in the correct order in the split second the bird is willing to stay still and give you a good view of both of its legs,” says Teunissen.

Brown fairy-wren
A female purple-crowned fairy-wren with food. Credit: © Niki Teunissen/AWC

In this study, the researchers found that subordinate wrens help out the most when they’re helping breeders that are related to them, or could be future mates.

“Based on all this evidence, we propose that cooperative helping behaviour can importantly be driven by direct or future benefits associated with specific social group members, but these benefits are generally overlooked in studies that aim to explain cooperative behaviours,” says Teunissen.

Next, Teunissen is looking at the “dispersal” of purple-crowned fairy-wrens: when they leave social groups.

“If we want to fully understand why these birds engage in cooperative breeding, we need to not only understand why birds help to raise others’ offspring in the group, but we also need to understand why these birds choose to stay in the group without breeding independently in the first place.

“So with my current research I am trying to figure out why some birds stay at home – some for many, many years even – whereas others choose to leave to try to settle elsewhere – close by or very, very far away – as a breeder.”

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