The effect of mycoplasmosis on carotenoid plumage coloration in male house finches.

Parasites are widely assumed to cause reduced expression of ornamental plumage coloration, but few experimental studies have tested this hypothesis. We captured young male house finches Carpodacus mexicanus in Alabama before fall molt and randomly divided them into two groups. One group was infected with the bacterial pathogen Mycoplasma gallicepticum (MG) and the other group was maintained free of MG infection. All birds were maintained through molt on a diet of seeds with tangerine juice added to their water as a source of beta-cryptoxanthin, the natural precursor to the primary red carotenoid pigment in house finch plumage. All males grew drab plumage, but males with MG infection grew feathers that were significantly less red (more yellow), less saturated, and less bright than males that were not infected. MG targets upper respiratory and ocular tissue. Our observations show that a pathogen that does not directly disrupt carotenoid absorption or transportation can still have a significant effect on carotenoid utilization.