Females may prefer elaborate sexual ornaments in males as these can be costly and may honestly indicate male viability. We used a wild population of the Blue-black Grassquit (Volatinia jacarina) in central Brazil to test whether more ornamented males have lower parasite loads (parasite-mediated sexual selection) and/or better body condition (condition-dependent sexual selection) compared with less ornamented males. We predicted that blue-black plumage coverage should be positively correlated to body condition (weight/tarsus) and negatively correlated to parasite load (Mallophaga lice). We found that blue-black plumage coverage of grassquit males was positively related to body condition score and negatively related to ectoparasite load. However, body condition was not correlated with ectoparasite load in these males. Our data are consistent with the hypothesis of sexual selection mediated by parasites as well as with the hypothesis of sexual selection mediated by body condition, indicating that nuptial plumage coverage can be an honest signal of male quality in Blue-black Grassquits, and could thus be used by females during mate choice.
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