Males with More Pheomelanin Have a Lower Oxidative Balance in Asian Barn Swallows (Hirundo rustica gutturalis)

Pheomelanin-based plumage pigmentation has been suggested to be an honest signal of individual quality to conspecifics. It has been hypothesized that oxidative stress is an important agent linking pheomelanic pigmentation to individual quality. Using the Asian barn swallow Hirundo rustica gutturalis, a wild passerine, we tested whether the pheomelanin pigmentation in the red throat patch of adult males, a sexually selected trait, is associated with the ratio between reduced and oxidized glutathione (RGSH/GSSG) as an indicator of current oxidative balance during the early breeding season. We found that males with a higher pheomelanin concentration in their throat feathers had a significantly lower RGSH/GSSG ratio (i.e., higher oxidative stress), but exhibited a better body condition, measured as residual body mass on body size, compared to males with a lower pheomelanin concentration. The total GSH level was not significantly related to the pheomelanin concentration. These findings suggest a negative association between the red pheomelanin pigmentation and oxidative balance during mating and reproductive efforts, which might be mitigated by high-quality males. Further research is needed to understand its actual damage and mitigation mechanism.