In a kind of post-workout glow, male crossbills that work harder when flying will grow redder plumage. The findings reveal that the processes that generate red feather pigments in the birds may be partly driven by the revving of the metabolic engines inside cells, called mitochondria.
Many animals acquire their bright colours from compounds in their diet. Male common crossbills (Loxia curvirostra) take in yellow carotenoid pigments from their food and use enzymes to convert the pigments into vibrant, red ketocarotenoids, stored in the feathers.
Scientists have long interpreted this blush of colour as a signal males use to advertise their good health to prospective mates. Foraging for food to make red pigments is a costly investment, says at the Spanish National Research Council in Madrid. The thinking is also that carotenoids may provide various health benefits to the males, like antioxidant protection, and so serve as a convincing signal of a red male’s fitness.
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“Only the highest quality animals should be able to express [the colour],” says Alonso-Alvarez, of the idea. “This was the mainstream in the last decades.”
But research in the 1950s showed that male crossbills kept in cages began growing only yellow feathers when they moulted. This, and suggesting that yellow-to-red carotenoid conversion may occur in the inner membrane of the mitochondria, made Alonso-Alvarez and his colleagues wonder if red feathers were partially a result of exercise and not just markers of diet and foraging skills.
Between October 2019 and February 2020, the team captured 295 male crossbills in central Spain, taking measurements of their colour, size and weight. To make flight a bit more physically and metabolically taxing, the researchers clipped some wing feathers from about half the crossbills. They also plucked feathers from all of the males’ rumps and released them back into the wild.
When a few dozen of the birds were recaptured in the following months, they had regrown their rump plumage – and the feathers growing back on the clipped birds were redder than those on their unclipped counterparts. The clipped birds also lost weight, which the researchers attribute to the added strain of flying while missing some important feathers.
Both findings suggest exercise is an important factor in feather redness. If colour was entirely a product of diet, then the birds that had lost weight – indicating they were having a harder time finding enough food to sustain themselves – would show less red pigmentation, not more.
Alonso-Alvarez and his team deduced that the production of red pigment is influenced directly by flight exertion. Red feathers may be a signal that depends “on the quality of the individual, but independent of resources and any cost of production”, says Alonso-Alvarez. This is akin to how call features in male frogs and deer are intrinsically tied to body size, which determines their evolutionary fitness. In nature, red colour may faithfully telegraph a crossbill’s healthy cellular metabolism, or capacity for vigorous flight.
“We might conventionally assume that birds with impaired flight are ‘lower quality’, but here they are growing redder feathers,” says at the University of Tulsa, Oklahoma, adding that study’s approach is “breathing fresh life” into work on carotenoid pigments.
There are other carotenoid conversion processes among birds, and Koch is curious to know what a similar study on those species might reveal.
Reference: bioRxiv,
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