Interactions between hemoparasite status and female age in the primary reproductive output of pied flycatchers.

The relationship between hemoparasite infection (measured just after egg laying) and primary reproductive output (laying date, clutch size and egg volume) was studied in female pied flycatchers, Ficedula hypoleuca, of different ages (2 years old vs. 4 or more years old). The hemoparasite (Haemoproteus balmorali and Trypanosoma spp.) prevalence increased with advancing female age. H. balmorali-infected females initiated egg laying earlier and laid larger clutches. There was no evidence that infected females laid smaller eggs or had a lower body mass after egg laying than non-infected ones. Only for H. balmorali-infected females was there an increase in clutch size between age groups. The present results suggest that primary reproductive output of females will depend on their health-dependent residual reproductive value. Only old females may be able to control a chronic or latent infection by hemoparasites and thereby invest more heavily in reproduction than younger but experienced females. Alternatively, high hemoparasite prevalence may be the cause of increased female primary reproductive output in old females.