Reproductive effort and blood parasites of breeding pied flycatchers: the need to control for interannual variation and initial health state

The association between reproductive effort, as assessed by experimentally altered clutch size, and prevalence of blood parasite infections was studied during two breeding seasons in the pied flycatcher (Ficedula hypoleuca). Only in the first study year (1998), enlarged clutches resulted in more fledged young and birds presumably responded to the intended experimental increase in reproductive effort by increasing their provisioning rates. Only on this study year, female body mass was negatively affected by the clutch size manipulation. For both sexes, no significant effect of experimental treatment on the prevalence of blood parasite infections (Trypanosoma spp., Haemoproteus balmorali) on day 13 after chick hatching was found in both study years. However, only in the first study year, when the intended increase in parental reproductive effort was achieved, we found a higher within female seasonal increase in Trypanosoma spp. prevalence for birds with enlarged clutches compared to those with control and reduced clutches. In this year, the Trypanosoma spp. prevalence was significantly higher than in the second study year. These results suggest that more than one year has to be covered, and that the knowledge of the premanipulative health status of parents can facilitate detection of the proposed trade-off between reproductive effort and immunocompetence.