Wild common crossbills produce redder body feathers when their wings are clipped
The animal signaling theory posits that conspicuous colorations exhibited by many animals have evolved as reliable signals of individual quality. Red carotenoid-based ornaments may depend on enzymatic transformations (oxidation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share the same biochemical pathways. Accordingly, the level of trait expression (redness) would directly reveal the efficiency of individuals metabolism and hence, the bearer quality in an unfalsifiable way. Different avian studies have described that the flying effort may induce oxidative stress. A redox metabolism modified during the flight could thus influence the carotenoid conversion rate and, ultimately, animal coloration. Here, we aimed to infer the link between red carotenoid-based ornament expression and flight metabolism by increasing flying effort in wild male common crossbills Loxia curvirostra (Linnaeus). In this order, 295 adult males were captured during winter in an Iberian population. Half of the birds were experimentally handicapped through wing feather clipping to increase their flying effort, the other half being used as a control group. The rump feathers were also plucked in all the birds to stimulate plumage regrown of a small surface during a short time-lapse. Thirty-four birds with fully grown rump feathers could be recaptured in the subsequent weeks. Among them, male crossbills with experimentally impaired flying capacity showed redder rump feathers than controls. This result would support the hypothesis that the flying metabolism influences the redox enzymatic reactions required for converting yellow dietary carotenoids to red ketocarotenoids. Summary statementCarotenoid-based colorations may depend on pigment enzymatic transformations linked to mitochondrial function. The flight metabolism could influence this. Here, common crossbills with clipped wings produced redder feathers than controls.