PLUMAGE COLORATION, DIFFERENTIAL ATTRACTION OF VECTORS AND HEMATOZOA INFECTIONS IN BIRDS

1. Haematozoa (blood parasites) infect the majority of passerine bird species, yet there is considerable variation among species in the prev alence of infections. The ecological and evolutionary causes and conse quences of this variation are not well understood. 2. Tests of Hamilto n & Zuk's hypothesis on parasites and sexual selection using haematozo a and 'plumage brightness' have produced mixed results. Debates about how to interpret these results and construct further tests have consid ered properties of the parasites and hosts, yet have virtually ignored the vectors of the parasites. 3. Many of the Diptera that transmit ha ematozoa show similar patterns of differential attraction to particula r colours. Thus, variation among avian species in prevalence of haemat ozoa may result from some plumage colours attracting more parasite vec tors. 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 vec tor-attraction hypothesis, but did not find the predicted positive rel ationship between plumage colours that attract vectors and haematozoa infections. 5. Further analysis of the parasite data, however, reveale d a problem for species-wide explanations of haematozoa parasitism. Re lative haematozoa prevalence in the same species sampled at different locations was not strongly correlated. This result suggests that any s pecies-wide explanations for variation in haematozoa parasites are, at best, extremely difficult to test, and, at worst, untenable.