Association between haematozoan infections and reproduction in the Pied Flycatcher

1. Parasites may affect breeding success of their host since they compete for the same resources as their hosts. Reproduction may also increase the susceptibility of a host to parasite infections owing to lowered resistance to parasites during breeding. 2. We studied the association between breeding performance and haematozoan parasite infection in the Pied Flycatcher (Ficedula hypoleuca) by using both natural data on reproduction and data from clutch size manipulations. 3. The most frequent blood parasites of the Pied Flycatcher in central Finland were Haemoproteus pallidus, Haemoproteus balmorali and Trypanosoma avium complex. 4. We did not find evidence that these haematozoan parasites have any debilitating effects on either reproduction or survival. The variation in reproductive effort did not seem to influence susceptibility to new blood parasite infections. 5. The intensity of Haemoproteus balmorali tended to increase in infected males as the brood size was artificially enlarged. Also, in females intensity of H. pallidus infection tended to increase with the level of clutch size manipulation. Thus, increased reproductive effort seems to debilitate the ability of Pied Flycatcher to control chronic infections. 6. Individuals with enlarged clutches/broods increased their reproductive effort at the expense of defence towards parasites. The cost of current reproduction may then be at least partly mediated by haematozoan infections.