Age-biased helminth infections uncover hidden migration effects

Host age can indicate where avian helminths are acquired, but age patterns may also reflect seasonality, cumulative exposure and parasite persistence. We analysed age-specific prevalence, abundance and infection intensity in our long-term dataset of individually examined birds from the Czech Republic. Helminth taxa were classified statistically according to age and seasonal effects, and, independently, trematodes were assigned a priori to biogeographic groups based on their life cycles and intermediate-host distributions. Among 177 taxa retained for statistical analysis, 34 showed a pronounced adult-biased effect (Category A), 9 showed neither a strong adult effect nor significant seasonality (Category B) and 134 showed significant seasonal effects without meeting the Category A criterion (Category C). The independent biological classification provided clearer evidence of geographic acquisition. Trematodes associated with wintering grounds in southern Europe or Africa occurred in 0.21% of first-year and 3.12% of adult host–taxon associations. In contrast, nonlocal trematodes completing their life cycles elsewhere in Europe or marine taxa occurred in 0.70% and 2.01%, respectively. In contrast, cosmopolitan trematodes and those completing their life cycle locally were more prevalent in first-year associations. Most qualifying nonlocal species showed adult-biased prevalence, although Psilotrema oligoon and Psilotrema simillimum showed the opposite pattern. Several biologically nonlocal taxa were assigned to Category B or C because age and sampling month were strongly confounded, showing that failure to detect an independent age effect does not imply local transmission. Age-structured helminth data, therefore, provide a useful signal of migration-mediated parasite acquisition when interpreted together with parasite life cycles, intermediate-host distributions, seasonality and persistence.