Melanin–based plumage coloration in the house finch is unaffected by coccidial infection

For most species of birds, ornamental plumage coloration may result from two types of pigments: carotenoids and melanins. Despite the fact that melanin pigments can be synthesized by birds from basic, amino acid precursors, while carotenoids cannot be synthesized by birds and must be ingested, melanin–based plumage coloration and carotenoid–based plumage coloration have often been treated as a single trait in investigations of the function and evolution of plumage coloration. Expression of carotenoid–based coloration is known to be dependent on condition, while the effects of individual condition have not been well–tested for expression of melanin–based coloration. In this study, we experimentally tested the effect of coccidial infection of the intestinal tract of male house finches during moult on expression of melanin–based plumage coloration. Coccidial infection had a significant negative effect on carotenoid–based coloration, but it had no significant effect on melanin–based feather coloration. Unlike carotenoid–based coloration, melanin–based coloration may be cheap to produce, and honesty of melanin–based coloration my require social mediation.