Plumage Coloration, Differential Attraction of Vectors and Haematozoa Infections in Birds

1. Haematozoa (blood parasites) infect the majority of passerine bird species, yet there is considerable variation among species in the prevalence of infections. The ecological and evolutionary causes and consequences of this variation are not well understood. 2. Tests of Hamilton & Zuk's hypothesis on parasites and sexual selection using haematozoa and `plumage brightness' have produced mixed results. Debates about how to interpret these results and construct further tests have considered properties of the parasites and hosts, yet have virtually ignored the vectors of the parasites. 3. Many of the Diptera that transmit haematozoa show similar patterns of differential attraction to particular colours. Thus, variation among avian species in prevalence of haematozoa may result from some plumage colours attracting more parasite vectors. 4. Using a comparative analysis that controlled for potentially confounding effects of phylogeny and variables already demonstrated to be correlated with plumage and parasite variation, we tested this vector-attraction hypothesis, but did not find the predicted positive relationship between plumage colours that attract vectors and haematozoa infections. 5. Further analysis of the parasite data, however, revealed a problem for species-wide explanations of haematozoa parasitism. Relative haematozoa prevalence in the same species sampled at different locations was not strongly correlated. This result suggests that any species-wide explanations for variation in haematozoa parasites are, at best, extremely difficult to test, and, at worst, untenable.