Reproductive investment and parasite susceptibility in the great tit

1. Reproduction and parasite defence are assumed to be costly activities for hosts, and therefore trade-offs might exist between reproduction and parasite defence, 2. Brood sizes were manipulated in a population of Great Tits (Parus major L.) to assess trade-offs between reproduction and parasite defence. Blood samples were taken from males and females during the late nestling phase, and parasite prevalence was compared among parents raising enlarged, reduced and control broods. 3. A higher prevalence of protozoan blood parasites dominated by Haemoproteus majoris was found among parents with enlarged broods, as compared with parents with control and reduced broods, respectively. Brood size manipulation did not affect parasite prevalence between the sexes or between age classes differently. 4. Mean parasite intensity (number of parasites per microscope field) among adult birds only infected with H. majoris was higher for parents with experimentally enlarged broods than controls and reduced broods, respectively. 5. These results support the hypothesis of a trade-off between parasite defence and reproduction.