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Early-Migrating Robins Carried More Ticks Than Late Migrants

European robin being examined during bird ringing
Research · Disease, parasites & microbiology

Researchers examining European robins (Erithacus rubecula) during autumn migration on the Baltic coast of northern Poland found that birds passing through early in the season carried significantly more ticks than later migrants. The study, first published on 29 May 2026, examined 198 robins and also tested whether small differences between the lengths of the birds’ left and right legs were associated with tick infestation.

Ticks were found on 49.5% of the robins. Here, prevalence means the proportion of birds carrying at least one tick, whereas tick load refers to the number of ticks carried by an individual bird. Of the ticks that could be identified, 98.5% were the castor bean tick (Ixodes ricinus), a widespread European species that parasitizes many vertebrates. When the researchers modelled tick abundance, migration period was the only variable with statistical support for predicting how many ticks a bird carried: early migrants had higher tick loads than birds captured later in autumn.

Body asymmetry did not explain tick load

The study was designed partly to test a proposed link between developmental instability and susceptibility to parasites. The researchers measured both tarsi and used the small left–right difference as a measure of fluctuating asymmetry. Such random differences between the two sides of the body can be used as an indicator of how consistently an animal developed, although they are not a direct measure of health or stress. They found no association between this asymmetry and either total tick load or the side of the body on which ticks occurred.

The authors describe the results as correlative. They conclude that moderate asymmetry in leg morphology may not influence susceptibility to ticks in migrating robins, while noting that other factors such as body condition, sex, age and length of stopover could still influence infestation.

Seasonal timing stood out

The strong relationship with migration period points instead towards seasonal processes in the host–parasite system. Tick activity, exposure before arrival at the study site and differences among birds moving through at different stages of the season could all vary through autumn. The study does not identify a single mechanism behind the higher tick loads of early migrants, but it establishes migration timing as the clearest pattern in this sample.

Sources and image credit

  1. Polakowski, M., Jankowiak, Ł., Remisiewicz, M., Rollins, R. E., & Goławski, A. (2026). The lack of a role of body asymmetry in shaping tick load in European Robins (Erithacus rubecula) during autumn migration. Ibis. https://doi.org/10.1111/ibi.70072Primary peer-reviewed study first published 29 May 2026. Among 198 European Robins sampled during autumn migration on the Baltic coast of northern Poland, tick prevalence was 49.5%; 98.5% of identified ticks were Ixodes ricinus, and early migrants carried significantly more ticks than late migrants. Tarsus asymmetry did not predict tick load.

Image: Calandrella / Wikimedia Commons — European Robin being ringed at Landsort Bird Observatory, Sweden, 8 October 2010; representative migration/ringing image, not a bird from the Polish study · CC0 1.0 · Source

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