Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus).

BACKGROUND

Climate warming is altering the seasonal timing of many ecological interactions. Migratory birds typically time their movements using photoperiodic cues, whereas the activity of ectoparasites such as ticks is largely temperature dependent. This difference in environmental drivers may create climate-induced phenological mismatches that reshape host-parasite interactions.

METHODS

We investigated tick infestation in migratory thrushes during late-autumn migration in Belgium, a period historically assumed to fall largely outside the main activity season of Ixodes ricinus. In addition, we screened ticks collected from these birds for tick-borne pathogens. Both sampling years (2022 and 2024) showed unusually mild autumn conditions.

RESULTS

284 redwings (Turdus iliacus) were examined, revealing exceptionally high tick infestation rates (72.9%), the highest reported for this species in Europe. Ixodes ricinus predominated, and screening of 698 ticks showed that 12.9% carried Borrelia spp., including multiple avian- and mammalian-associated genospecies as well as Borrelia miyamotoi. Infected ticks were present on nearly one-third of birds. First calendar-year birds carried significantly higher tick loads than older individuals. Extrapolation to the European redwing population suggests that tens of millions of infected ticks may be transported southward each autumn.

CONCLUSIONS

These findings indicate that late-autumn migration can coincide with substantial tick activity under mild climatic conditions, creating a previously overlooked seasonal window for parasite dispersal. Because migration timing is relatively stable whereas parasite activity is climate sensitive, continued warming is likely to strengthen this phenological overlap.