Spring reproductive success influences autumnal malarial load in a passerine bird

Abstract

Although avian haemosporidian parasites are widely used as model organisms to study fundamental questions in evolutionary and behavorial ecology of host-parasite interactions, some of their basic characteristics, such as seasonal variations in within-host density, are still mostly unknown. In addition, their interplay with host reproductive success in the wild seems to depend on the interaction of many factors, starting with host and parasite species and the temporal scale under study. Here, we monitored the intensities of infection by two haemosporidian parasites (i.e. Plasmodium relictum and P. homonucleophilum) in two wild populations of great tits (Parus major) in Switzerland over three years, to characterize their dynamics. We also collected data on birds reproductive output (i.e. laying date, clutch size, fledging success) to determine whether they were associated with infection intensity before (winter), during (spring) and after (autumn) breeding season. Both parasite species dramatically increased their within-host density in spring, in a way that was correlated to their parasitaemia in winter. Infection intensity before and during breeding season did not explain reproductive success. However, the birds which fledged the more chicks had higher parasite burdens in autumn, which were not associated with their parasite burden in previous spring. Our results tend to indicate that high haemosporidian parasite loads do not impair reproduction in great tits, but high resource allocation into reproduction can leave birds less able to maintain low parasitaemia over the following months.