Flight patterns of a migratory wader over the North Sea reveal implications for interactions with offshore wind farms

With expected increases in global offshore wind power capacity and increased turbine numbers over the next decade, there is an urgent need to assess how migrant birds traverse marine areas. Offshore wind farms pose a direct collision risk to migrating waders, yet we lack understanding of how environmental processes such as wind and weather conditions influence seasonal movements and flight activity, and how this may govern vulnerability to collision during migration. We tracked a near-threatened migratory wader (shorebird), Eurasian curlew, Numenius arquata (335 birds) during migration using GPS telemetry between 2014 and 2025, across the North Sea, an area central to expansion of offshore wind developments in Europe. In order to better inform potential collision risk, we examine four components of migratory flight behaviour: (i) spatial area use, (ii) timing of passage through the study area, and two measures of movement characteristics: (iii) flight height and (iv) ground speed. We examine variation in these parameters over different seasons (spring and autumn), time of day, and between males and females, and the influence of environmental drivers, including directional wind speed, precipitation, cloud cover, nocturnal illumination and relative humidity. Migration was most frequent in the southern North Sea, being coastal or covering shorter open water distances between the UK and the European continent. Peak passage intensity occurred during early-evening 20–21:00 and early morning 03–04:00, typically during low-light levels. Curlew generally flew quite low (median 108 m, mean 338 ± 557 m) with mean ground speeds of 17.2 ± 5.7 m/s (median 16.1 m/s). Patterns varied seasonally and diurnally, with the lowest flight heights in autumn and at night ca. 20:00–04:00. Ground speeds were greater during spring than autumn but were no different between males and females. Higher flight altitudes were attained and faster ground speeds recorded in faster tailwind speeds (with opposite patterns for head and crosswinds). Lower flight altitudes were recorded during periods of higher precipitation. This study characterised migration flight patterns of Eurasian curlew over the North Sea section of the East Atlantic flyway. Flight height and ground speed patterns over increasing tailwind speed were in keeping with wind-assisted support. Given predominantly west-south-westerly prevailing winds, tailwinds were more likely in spring. Airspeed was also greater for spring than autumn, perhaps indicating further ecological drivers related to seasonal migratory strategy. With the rapid expansion of offshore wind farm development in this region, these results will enable more accurate assessment of collision vulnerability and development of better mitigation strategies.