Blood Parasite Infections and Reproductive Performance in a Multi-Host Avian System

Blood parasites are widespread pathogens of wild birds, yet their reproductive consequences remain poorly understood. We investigated associations between blood parasite infections and reproductive performance in three passerine species breeding sympatrically on Gotland, Sweden: the resident great tit (Parus major) and blue tit (Cyanistes caeruleus) and the migratory collared flycatcher (Ficedula albicollis). The study was based on data from 181 nests and 1201 nestling observations. Adult birds were screened for Plasmodium, Haemoproteus, and Trypanosoma infections using next-generation sequencing, and infection intensity of malaria parasites (Plasmodium/Haemoproteus) and Trypanosoma was quantified using quantitative PCR. We tested whether parental infection status and parasite intensity were associated with reproductive traits measured at the nest level (hatching date, clutch size, and brood size) and nestling level (body mass, growth, and condition). Associations between infections and reproduction differed among parasite groups, parental sexes, and host species, with some of the strongest and most consistent patterns involving Haemoproteus infections. Most significant relationships were detected for nestling traits rather than breeding parameters, and a substantial proportion of associations remained significant after false discovery rate (FDR) correction. Both positive and negative associations were observed, and significant interactions between female and male infection status indicated that offspring performance depended on both parents' infection status. Moreover, analyses based on infection status and parasite intensity revealed partly different patterns of association with reproductive traits. Overall, these results demonstrate that the reproductive consequences of blood parasite infections are parasite-specific and context-dependent.