Importance of infection of haemosporidia blood parasites during different life history stages for long‐term reproductive fitness of collared flycatchers

The interaction between birds and haemosporidia blood parasites is a well-used system in the study of parasite biology. However, where, when and how parasites are transmitted is often unclear and defining parasite transmission dynamics is essential because of how they influence parasite-mediated costs to the host. In this study, we used cross-sectional and longitudinal data taken from a collared flycatcher Ficedula albicollis population to investigate the temporal dynamics of haemosporidia parasite infection and parasite-mediated costs to host fitness. We investigated host-parasite interactions starting at the nestling stage of the bird's life-cycle and then followed their progress over three breeding attempts to quantify their fitness - measured as the number of offspring they produced that recruited back into the breeding population. We found that the majority of haemosporidia blood parasite infections occurred within the first year of life and that the most common parasite lineages that infected the breeding population also infected juvenile birds in the natal environment. Moreover, our findings suggest that collared flycatcher nestlings in poorer condition could be at a higher risk of haemosporidia blood parasite infection. In this study, only female and not male bird fitness was adversely affected by parasite infection and the cost of infection on female fitness depended on the timing of transmission. In conclusion, our study indicates that in collared flycatchers, early-life is potentially important for many of the interactions with haemosporidia parasite lineages, and evidence of parasite-mediated costs to fitness suggest that these parasites may have influenced the host population dynamics.