Life cycle complexity shapes parasite sharing amongst migratory and resident hosts

Migratory animals are potentially important parasite dispersers. However, understanding the impact of host migration on parasite distributions is challenging because variation in parasite life histories can mediate dispersal potential. Furthermore, studies of parasite dispersal via host migration often compare parasites in different species of migrant and resident hosts, despite differences in host phylogeny, ecology, and exposure history, which can complicate ascertaining the impact of migration. The Tasmanian population of silvereye (Zosterops lateralis lateralis) exhibits partial migration. Some members of the population overwinter on the Australian mainland, where they co-occur with residents of another silvereye subspecies (Z. l. cornwalli). Utilising targeted PCR, metabarcoding and high-throughput sequencing, we characterise a range of parasite genera from blood and faecal samples in Tasmanian resident, migrant and mainland resident birds. Greater parasite richness was documented in migrant individuals, possibly reflecting exposure to varied parasite faunas at stopovers or during overwintering. The increased richness was primarily driven by parasites with simple life cycles. Reduced sharing of parasites with complex life cycles amongst migratory and resident host populations may be explained by ecological constraints in establishing transmission, such as vector or secondary host availability. Our study exemplifies the utility of molecular methods for characterising multi-parasite systems and makes use of a single host species to provide insights on patterns of parasite distributions that result from migratory movements. Knowing how parasite life histories constrain or promote dispersal via regular host migration will be essential for predicting future patterns of pathogen dispersal under climate change and shifting migratory regimes.