Seasonal migration reorganizes host communities but not their haemosporidian infection dynamics

Host community composition can strongly influence multi-host parasite dynamics; however, whether predictable seasonal reassembly driven by animal migration reshapes local infection dynamics remains unclear. Across four annual cycles, we monitored passerine communities and haemosporidian parasite infection dynamics across five sites in southern Chile spanning interspersed forest and riparian wetland habitats seasonally visited by the long-distance Neotropical migrant Elaenia albiceps. Using molecular lineage identification and blood smear microscopy, we distinguished host species sustaining effective parasite circulation from hosts infected through dead-end spillover events and quantified each host’s contribution to multi-host lineages by decomposing abundance, prevalence, and gametocytemia. The spring arrival of E. albiceps dramatically reorganized forest communities, becoming the numerically dominant species during the breeding season, whereas riparian wetland communities remained comparatively stable under the year-round dominance of resident Tachuris rubigastra. Despite this significant seasonal reordering, local haemosporidian dynamics remained largely unchanged. Multi-host Plasmodium and Leucocytozoon lineages circulated within a subset of ecologically proximate forest residents, with individual species contributing disproportionately to each lineage through different host asymmetries. In contrast, E. albiceps maintained a migrant-associated Haemoproteus lineage but showed little evidence of acquiring resident-associated lineages, and the few cross-infections between migratory and resident hosts lacked transmissible stages, consistent with dead-end spillovers. Thus, ecological opportunity defined which species were exposed to circulating lineages, whereas physiological compatibility determined whether the infections reached transmissible stages. These results demonstrate that migration can seasonally reorganize host communities without restructuring local parasite circulation, which remains confined to ecologically proximate residents through opportunity and compatibility barriers that may be obscured in broad-scale lineage-sharing analyses.