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The prevalence and intensity of haematozoan parasites, with special emphasis on Haemoproteus coatneyi, was studied in relation to male plumage coloration, and reproductive success in a population of yellowhammers (Emberiza citrinella). Parasite prevalence and intensity were found to vary with time in season, emphasising the need to take samples during times of developed infection. Males were found with a peak intensity during the early breeding period and females slightly later, during the nestling period. The hypothesis of parasite-mediated sexual selection, according to which bright plumages have evolved in relation to degree of parasite infection, was tested. Male colour was found to reliably reveal the intensity of parasites during the breeding period. Males with high parasite intensity produced fewer fledglings. In spite of the negative correlation between parasite load and colour, males with more colour did not produce more offspring. No cost of parasites was found in females, and pairing was not assortative with respect to parasite infection. However, contrary to the hypothesis, no relationship between male colour and number of fledglings was found. Conclusively, some support was found for the hypothesis of parasite-mediated sexual selection, i.e. bright male plumages in the yellowhammer may thus have evolved or is maintained as a signal of level of parasite infection since a reduced reproductive success may be a cost imposed by high parasite load.
Oikos © 1995 Nordic Society Oikos