Reproductive Effort Influences the Prevalence of Haematozoan Parasites in Great Tits

1. The influence of reproductive effort on host susceptibility to parasitism was examined in great tits, Parus major, by comparing the prevalence of haematozoan parasites with respect to clutch size in male and female parents. 2. Observational and experimental studies were conducted. Observational studies documented the relationship between clutch size and parasite prevalence in males and females in unmanipulated nests. Reproductive effort was manipulated by exchanging complete clutches between pairs of nests during incubation. Parents experienced a maximum manipulation of ± 5 eggs. 3. Observational studies showed that the prevalence of parasites was higher in females than males. The prevalence of parasites in males increased with both increasing clutch size and increasing age