Nowhere to escape: the cross-age avian haemosporidian exposure of migrants in northeast China

Pathogen infection has been progressively recognized as a potential driver of animal migration, and the infection pattern in a certain host may be related to its migration status. Nestling viability can be a crucial life stage in the evolution of the migration cycle but have been irregularly studied to date. In this study, we tested the 'migratory exposure' hypothesis by comparing the prevalence and diversity of avian haemosporidian parasites in the breeding area between two dominant species including adults and nestlings: yellow-rumped flycatcher Ficedula zanthopygia, which is a summer migratory species, and Japanese tit Parus minor, which is a local resident species. The yellow-rumped flycatcher harboured more diverse parasite lineages than the Japanese tit, which is in line with the expectation of the 'migratory exposure' hypothesis, while nestlings present a similar but more applicable pattern. Among the 40 unique identified parasite lineages, only one was shared between host species and three between age classes. Nestlings suffered a high parasite diversity with plenty of age-specific lineages in the breeding area, mainly Leucocytozoon, which may suggest an extraordinary selection pressure in early life-stages during migration. However, the most common lineages in adults were not detected in nestlings, and the prevalence of infection is significantly lower in nestlings than in adults. Our results suggest that migratory birds may have suffered from more frequent parasite infections during migration, but escaped from more virulent lineages at the same time. The difference in susceptibility and parasite assembly between adults and nestlings in migrants may result from an intricate interaction along the evolution of life history. Our investigations have revealed the significance of avian haemosporidian parasite infection patterns in nestlings in breeding areas, and provide a novel insight into the driving forces of migration.