Haematozoan infections in the Eurasian kestrel: effects of fluctuating food supply and experimental manipulation of parental effort

The influence of parental effort on susceptibility to parasitism was invest igated experimentally in the Eurasian kestrel (Falco tinnunculus) in Finlan d. Parental effort was manipulated by either enlarging or reducing broods b y 1-2 young, while unmanipulated broods served as controls. This was done d uring 3 breeding seasons, during which the densities of the major prey of k estrels (voles) were relatively low, high and intermediate, respectively. T wo, taxon specific, methods were applied for determining the prevalence (pe r cent individuals infected) of extracellular Trypanosoma spp. and intracel lular Haemoproteus spp. infection. Blood samples were taken from females du ring the incubation phase, and from both parents during the mid-nestling ph ase. Trypanosoma was more prevalent when food was less available. Furthermo re, the increase in the prevalence of Trypanosoma among females was most pr onounced during the poor vole year. In contrast, Haemoproteus infection see med not closely related to annual supply of main prey. Manipulations of par ental effort were related to parental infection, but the effect differed be tween genders. The prevalence of Trypanosoma in males (main provider of you ng) increased with experimental brood size, and there was an interaction be tween food supply and brood size manipulation so that the difference in pre valences between reduced and enlarged broods increased with decreasing food supply. Among females, no such an interaction was found. Instead, the effe ct of brood enlargement on Trypanosoma prevalence of females was apparent i n the pear of relatively high vole densities only. Manipulation of brood si ze did not have clear effects on Haemoproteus infection in either gender. T hese results support the idea that increased parental effort may make hosts susceptible to haematozoan infection, and are the first to suggest that re source levels (food supply) can modify the susceptibility, indicating that the magnitude of reproductive costs can be attributed to Variation in envir onmental conditions.