High parasite load in house finches (Carpodacus mexicanus) is correlated with reduced expression of a sexually selected trait

Independent of age or geographic variation, males of many species of birds exhibit dramatic variation in the expression of elaborate secondary sexual characters. ''Good genes'' models of sexual selection propose that males with relatively low resistance to parasites suffer high parasite loads that inhibit their ability to express these characters fully. In turn, variation in such characters may reliably indicate male quality and may be used by females to choose males. This is the first study to monitor (via mark-recapture) the long-term effects of parasites on color and growth of plumage in individual birds. Specifically, we used house finches, Carpodacus mexicanus, a sexually dimorphic species in which females are known to prefer more brightly plumaged males for mating, to test the hypothesis that high parasite load in males is correlated with poor physiological condition and reduced development of male secondary sex characters. Our results clearly demonstrate that both ectoparasitic feather mite (Proctophyllodes sp.) infestations and endoparasitic avian pox viral infections during molt are correlated with poor physiological condition and reduced development of bright male plumage during the same molt period, thus supporting good genes models.