From release to migration: early ranging behaviour of headstarted Eurasian Curlews (Numenius arquata)

Abstract Headstarting is increasingly used as a conservation intervention for the Eurasian Curlew ( Numenius arquata ), yet evidence on the early post-release movement ecology of captive-reared juveniles remains limited. We analysed the post-release ranging behaviour of headstarted curlews using GPS tracking data collected over 5 years, from 2017 to 2021, in Poland. A total of 62 individuals were tagged, of which 34 were included in analyses of post-release movements, including 20 individuals that provided GPS tracks covering the entire pre-migration period and were therefore used for home-range analyses. On average, birds departed on autumn migration 28.9 ± 9.7 (mean ± SD) days after release, and no relationship was found between release date and departure timing. During the pre-migration period, the mean home-range size estimated using kernel density estimation was 188.6 ± 96.1 ha, based on the 90% isopleth, and the mean core area size was 29.8 ± 14.9 ha, based on the 50% isopleth. Spatial and habitat-related predictors did not explain variation in home-range or core area size, whereas temporal factors were important: both the duration of the pre-migration period and the release date were positively associated with spatial extent. Post-release movement behaviour was variable: half of the birds undertook exploratory movements, and more than one quarter relocated to one or more subsequent post-release areas, sometimes covering distances exceeding 200 km. Generalised additive mixed models revealed pronounced non-linear changes in ranging behaviour, with both distance from the release site and daily distance travelled increasing during the first weeks after release and peaking around day 18. Movement activity followed a clear bimodal daily pattern, with peaks in the morning and afternoon. These results demonstrate that headstarted curlews develop complex movement patterns before migration, highlighting the importance of temporal dynamics, individual variability and suitable release-site selection when planning headstarting programmes.