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Nearly Three-quarters of Migrating Redwings Carried Ticks During Two Mild Belgian Autumns

Redwing standing on grass with its head and body clearly visible
Research: Disease, parasites & microbiology, Ecology

Nearly three-quarters of redwings (Turdus iliacus) examined during late-autumn migration in Belgium carried ticks. Researchers checked 284 birds captured in November 2022 and 2024 and found ticks on 72.9%, among the highest infestation levels reported for the species in Europe over the past seven decades.

Two mild autumns extended the overlap between migration and tick activity

The timing is central to the result. Long-distance autumn migration in many birds is strongly linked to photoperiod, whereas the host-seeking activity of ectothermic ticks is highly temperature dependent. The European autumns of 2022 and 2024 were the third- and fifth-warmest in the comparison used by the study, at 1.25 °C and 0.98 °C above the 1991–2020 average. Mild conditions can therefore extend the period in which active ticks overlap with southbound birds — a phenological overlap between biological processes driven by different environmental cues.

The castor bean tick (Ixodes ricinus) dominated the material, mainly as larvae and nymphs; no adult I. ricinus were found on the thrushes, consistent with adults of this species feeding mainly on larger mammals. The bird-specialized tick Ixodes frontalis was also present and included feeding adults. First-calendar-year redwings carried more ticks than older birds, but young birds were not clearly more likely than older birds to have at least one tick. The authors note that the higher loads in young birds could reflect greater exposure or weaker acquired resistance, but the data cannot distinguish between those explanations.

Borrelia DNA was detected in 12.9% of screened ticks

Of 698 ticks screened for pathogens, 90 — 12.9% — were positive for Borrelia. Six known genospecies within the Borrelia burgdorferi sensu lato complex were detected: the bird-associated B. turdi, B. garinii and B. valaisiana, together with the mammal-associated B. burgdorferi sensu stricto, B. spielmanii and B. afzelii. The relapsing-fever spirochaete Borrelia miyamotoi was found in ten ticks. None of the screened ticks tested positive for tick-borne encephalitis virus (TBEV).

At the host level, Borrelia-infected ticks occurred on nearly one-third of the redwings carrying I. ricinus. The presence of bacterial DNA in a feeding tick does not automatically show that every tick will transmit viable bacteria to its next host, or that the redwing itself was systemically infected. The birds were not directly tested for infection, and the authors distinguish carriage and dispersal of infected ticks from experimentally demonstrated vector or reservoir competence.

The Europe-wide estimate depends on a strong assumption

The authors estimate that Europe’s autumn population includes 16–28 million mature redwings and 120–210 million first-calendar-year birds. If infestation and infection levels similar to those measured at the Belgian stopover applied broadly, redwings could have transported nearly 100 million infected ticks southward in autumn 2024. They explicitly caution that this extrapolation assumes one Belgian stopover was representative of migrating redwings across Europe.

The study therefore identifies a seasonal mechanism by which warming can reshape host–parasite interactions. Redwing migration timing is comparatively stable because of its photoperiodic control, while tick activity can extend later into autumn as temperatures remain mild. Continued warming could consequently enlarge the late-autumn window in which migrating thrushes encounter active ticks and disperse them and their pathogens over long distances.

Sources and image credit

  1. Heylen, D. J. A., Ledegen, I., Grimon, M., Reteng, P., Hübschen, J. M., & Krawczyk, A. I. (2026). Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus). Parasites & Vectors, 19, 397.Primary peer-reviewed open-access article published 12 September 2026; full text checked. Ticks were found on 72.9% of 284 redwings captured during late-autumn migration in Belgium. Of 698 screened ticks, 12.9% were Borrelia-positive and none were positive for TBEV. The authors projected that redwings could have transported nearly 100 million infected ticks southward in autumn 2024, but explicitly caution that this extrapolation assumes one Belgian stopover was representative of the wider European migration.

Image: Redwing (Turdus iliacus) on Heligoland, Germany, 28 October 2012. Photo: Andreas Trepte / www.avi-fauna.info / Wikimedia Commons. CC BY-SA 2.5. Representative species image; not a bird from the Belgian tick study. · CC BY-SA 2.5 · Source

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