Fairywren colours intensify through breeding season

Male Red-backed Fairywrens (right) have an intense scarlet patch on their backs in breeding plumage; females (left) are rather plainer. Photo: The Auk/S Lantz.
Male Red-backed Fairywrens (right) have an intense scarlet patch on their backs in breeding plumage; females (left) are rather plainer. Photo: The Auk/S Lantz.
Male Red-backed Fairywrens replenish their lurid feathers throughout the breeding season.

Males of the Australian songbird species attract mates with bright red feather patches, and both colour and timing matter: the earlier they moult from their dull non-breeding plumage into showy  black and red, the more attention they get from the females.

However, Tulane University, Louisiana, researchers Samantha Lantz and Jordan Karubian discovered that, instead of facing a trade-off between bright and long-lasting colours, males are able to use 'adventitious moult' – that is, where odd feathers are replaced randomly or accidentally – to intensify their coloration over the course of the breeding season.

Males that moulted into red-backed plumage during the non-breeding season started out with the same intensity of red as those that moulted later. However, Lantz and Karubian showed that these early-moulting males actually increased in redness as time went on by replacing their original red feathers with even brighter ones, so that during the breeding season they were even more red than later-moulting males.

Early-moulting birds change into their breeding plumage before the onset of the rainy season, and adventitious moulting may let them take advantage of increased food availability when the rains arrive. Because adventitious moult is hard to document, requiring recapturing the same birds throughout the year, it may be more common that ornithologists realise.

Lantz and Karubian carried out their study with a colour-banded population of Red-backed Fairywrens in Australia’s Northern Territory in 2012-14, capturing individuals during successive non-breeding and breeding seasons to track the changes in their plumage.

“Thinking about it, it actually makes a lot of sense that the wrens would need to replace their feathers periodically, as life in the Northern Territory can be pretty tough,” says Lantz. “The winter is very hot and dry with lots of fires, and the summer is the wet season when it rains almost every day. I’ve been caught in a number of thunderstorms there, and I am thankful to be able to retreat under a roof. It’s hard what to imagine it must be like for the wrens, when some of the raindrops are almost as big as they are!”

“The authors have gathered very intriguing evidence that birds can adjust their plumage colour after the primary moulting period,” adds Dan Baldassarre of the University of Miami, an expert in sexual signals in fairywrens. “This is a very exciting discovery, because moult is typically thought of as a one-and-done event. These results suggest, rather, that moult may be more flexible, allowing individuals to modify their sexual signals throughout the season.”

Lantz, S M, and Karubian, J. 2016. Male Red-backed Fairywrens enhance a plumage-based signal via adventitious molt. The Auk 133: 338-346. 
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