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22 of 24 Annual-Cycle Measures Were Repeatable in GPS-Tracked Eurasian Curlews

Eurasian curlew flying over boreal landscape
Research · Ecology

A long-term GPS study found that individual Eurasian curlews (Numenius arquata) repeated much of their annual routine from year to year. The full dataset contained 94 adults tracked between 2014 and 2021, with some individuals followed for as many as seven consecutive years. Researchers analysed 24 spatial and temporal measures across breeding, wintering and migration. Twenty-two showed significant repeatability.

Repeatability asks whether the same individual retains its own characteristic timing or spatial pattern across years relative to the differences among birds. A value close to 1 therefore indicates a strong and stable individual pattern, not that a bird follows exactly the same path or date every year. The number of birds differed among analyses because each measure required at least two repetitions of the relevant annual-cycle stage: 94 birds contributed repeated wintering periods, 93 repeated breeding periods, 40 at least two complete spring migrations and 67 at least two complete autumn migrations.

Breeding and wintering sites showed the strongest fidelity

The strongest repeatability occurred in the locations of breeding and wintering sites, with values close to 1. The straight-line distance between an individual’s breeding and wintering sites was also highly repeatable, whereas the distance actually flown varied more. Migration routes were not identical between years, but consecutive routes from the same bird were substantially more similar to one another than routes from different birds in the same spatial group.

Repeatability estimates for Eurasian Curlew spatial, migration and stopover measures in spring and autumn
Repeatability estimates across breeding, wintering, migration and stopover measures; filled points are statistically supported estimates and open points are not. Figure 6 from Schwemmer et al. (2026), Communications Biology, CC BY 4.0.

Spring migration was more repeatable than autumn migration

Spring migration was generally more consistent than autumn migration. Migration duration had a repeatability of about 0.6 in spring and 0.4 in autumn. The two measures without statistically supported repeatability were the date birds left the breeding area in autumn and the number of stopovers during autumn migration.

The authors suggest that variable breeding success may help explain the flexible autumn departure date: birds that fail to breed can leave earlier than birds that complete a breeding attempt. This interpretation is biologically plausible and supported by previous curlew work, but the repeatability analysis itself did not experimentally test the cause.

The broad pattern suggests strong annual routines and comparatively limited short-term flexibility. The authors caution, however, that high repeatability is not the same as proof that curlews cannot adapt. The study does not establish how individuals or future generations might respond to stronger or longer-lasting environmental change.

Sources and image credit

  1. Schwemmer, P., Donnez, M., Mercker, M., Garthe, S., Boschert, M., Düttmann, H., Elts, J., Fartmann, T., Fiedler, W., Jiguet, F., Kämpfer, S., Korniluk, M., Kruckenberg, H., Krupiński, D., Marja, R., Piha, M., Rousseau, P., Rupprecht, V., & Bocher, P. (2026). High annual-cycle repeatability suggests low flexibility to environmental changes in a near-threatened migratory shorebird. Communications Biology, 9, 736. https://doi.org/10.1038/s42003-026-10371-0Primary peer-reviewed open-access article published 30 May 2026 under CC BY 4.0. The study analysed 24 annual-cycle measures in 94 GPS-tagged adult Eurasian Curlews followed for at least two repeated periods and up to seven consecutive years; 22 measures were significantly repeatable, with the strongest repeatability in breeding- and wintering-site locations.

Image: Estormiz / Wikimedia Commons — Eurasian Curlew in flight at Simo, Finland, 15 July 2011; representative species image, not a study bird · CC0 1.0 · Source

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