Ecology of Red-winged Blackbird parasitism by haematozoa

We sampled haematozoa in Red-winged Blackbirds (Agelaius phoeniceus) over three consecutive breeding seasons in eastern Ontario to identify factors that affect the reliability of quantifying parasites as this pertains to testing Hamilton and Zuk's hypothesis of sexual selection. Parasite prevalence and intensity varied seasonally, indicating that for samples to be reliable they should not be taken either early or late in the breeding season, and analysis should be limited to prevalence (presence versus absence). Among males (but not females), parasite prevalence increased with age, indicating that all individuals are not exposed to parasites early in life, and therefore tests of the "parasite" hypothesis of sexual selection are best restricted to older individuals. Resampling of known individuals within and between years showed that some individuals changed from positive to negative. Prevalence estimates also varied by year, indicating that reliable estimates of prevalence for a species, necessary for interspecific tests of the parasite hypothesis, will require extensive sampling. Overall, parasite prevalence did not vary with habitat. Parasites were detectable in only 1 of 119 nestlings sampled, indicating either lack of exposure or inadequate time for infections to reach detectable intensities. In either case, it is not possible to test the assumption that parasite resistance is heritable using blood samples from nestlings. Although males and females were parasitized by the same array of parasites, the distribution of specific parasites differed significantly (males were more heavily parasitized by leucocytozoids). If males and females generally differ in either exposure or resistance to specific parasites, the coevolutionary dynamics between host and parasite and the implications for sexual selection will require reassessment.