Darwin1 proposed that secondary sexual characters, such as the long tails and bright plumage of many birds, evolved because females use them as cues in male choice. The question of why females should prefer males with such apparently deleterious characters is currently the subject of vigorous debate2-7. Hamilton and Zuk8 suggest that females use secondary sexual characters to assess a male's ability to resist parasites. A prediction of this hypothesis is that male brightness should correlate positively with parasite load across species, and the only evidence advanced in support of the model is Hamilton and Zuk's8 finding that such a correlation exists across North American passerines. But interspecific correlations of this sort can result from phylogenetic associations through common descent or from independent associations with some confounding variable, such as an aspect of behaviour or ecology9-13. Here, I use data on European passerines and an enlarged data set on North American passerines to demonstrate positive relationships between male brightness and parasite prevalence which remain when the effects of taxonomic, behavioural and ecological variables are removed.
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