Latitudinal variation in seasonal migration of Eurasian curlews Numenius a. arquata breeding in Great Britain: breeding failure advances departure timing in southern populations

Abstract Background Migration plays a crucial role in the annual cycle of migratory birds. The Eurasian curlew Numenius arquata arquata , a species of high conservation concern in Great Britain, exhibits a wide range of migratory behaviours across its western distribution, from resident and short-distance movements to long-distance migrations along the East Atlantic Flyway. However, migratory behaviours of British-breeding curlews remain poorly studied. In particular, the ecological and life-history factors underlying variation in migration, such as timing and duration, remain unclear in this declining species. Methods We used GPS-tracking data from 80 Eurasian curlews breeding across a 703-km latitudinal gradient in Great Britain to examine how migration behaviours are influenced by sex, breeding-site latitude, local weather conditions, individual repeatability and reproductive success. Statistical analyses assessed relationships between these variables and timing of migration events. Results We found various sex-specific migration behaviours indicating contrasting selection pressures. Curlews breeding at higher latitudes (Scotland and northern England) left wintering sites earlier and departed breeding sites later than birds breeding at lower latitudes (southern England), although both groups arrived at breeding sites at similar times. Breeding-site latitude also affected migration distance, duration and speed in both pre- and post-breeding migration. Although migration timing was consistent within individuals across years, timing also varied with local weather conditions. Warmer temperatures were associated with earlier pre-breeding departures and arrivals, whereas post-breeding movements occurred later under warmer, wetter and calmer conditions. Timing of post-breeding migration was related to reproductive outcomes: successful breeders departed later, likely reflecting parental care of broods. This effect was driven by southern birds, which showed overall lower breeding success and departed earlier than northern birds. Conclusions Our main results reveal clear individual and latitudinal variation in curlew migration behaviour, with reproductive success influencing migration timing. These findings underscore the need to integrate life-history context into migration research, as spatial variation in breeding success, influenced by land-use and management, and climate change, can produce systematic differences in migratory timing. Such differences may influence subsequent habitat use across the annual cycle, with potential consequences for population dynamics and conservation of declining migratory wader species such as Eurasian curlews.