EFFECTS OF BLOOD PARASITES ON SEXUAL AND NATURAL-SELECTION IN THE PIED FLYCATCHER

The occurrence of the blood parasites Haemoproteus and Trypanosoma in the pied flycatcher, Ficedulla hypoleuca, was examined to test current hypotheses that parasites reduce the expression of secondary sexual t raits, mating success, breeding success, and survival of infected indi viduals. The results showed that there was no significant relationship between male plumage brightness and Trypanosoma infection, but males infected with Haemoproteus tended to be brighter than uninfected males , partly because first-year males were less often infected than older males. Polygynous males did not have fewer parasites than monogamous m ales. Females did not choose uninfected males among those they had sam pled. Clutch size and laying date were not related to female infection status, and the number and quality of nestlings was not related to pa rasite infections of either male or female parent. The ability of male s to provide parental care was not related to their infection status. The return rate from one breeding season to the next of infected males was not lower than that of uninfected males. The lack of correlations between parasites and male plumage colour and female mate choice appa rently do not support the Hamilton-Zuk hypothesis of sexual selection. However, the absence of demonstration of any negative effects of para sites suggests that infection status may not be a direct measure of pa rasite resistance, or the degree to which the host suffers. Instead, t he results support the alternative view that infected individuals have demonstrated their ability to survive and to cope with the parasites, while uninfected individuals are probably not yet tested for their re sistance., This points to problems in using parasite prevalences and d istributions, al least of some protozoan blood parasites, for tests of Hamilton-Zuk hypothesis, even though this was done in the first test by Hamilton and Zuk and by many later researchers.