The vertical dimension of bird migration: spatial and temporal variation in flight altitude of migrating Eurasian woodocks (Scolopax rusticola)

While the spatial and temporal variation of bird migration has been extensively studied in two dimensions, our understanding of fine-scale individual behavioural patterns in the vertical dimension remains limited. We used satellite telemetry to investigate spatial and temporal patterns of flight altitude of migrating Eurasian woodcocks (Scolopax rusticola), a nocturnal, forest-specialist species that undertakes long-distance migrations of up to 6000 km largely across Eurasian landmasses. We deployed GPS-GSM transmitters on individuals wintering or migrating along the Italian Peninsula and in several Mediterranean islands and tracked their migratory movements to and from their breeding grounds. Based on the distribution of ground speeds, we identified 566 night-time locations with active migration (ground speed >18 km/h) and reliable altitudinal/positional data (>4 satellites), 527 during pre-breeding and 39 during postbreeding migration, from 67 individuals. The median flight speed was 61 km/h, while the median flight altitude was 370 m a.g.l., confirming that woodcocks tend to migrate at relatively low altitudes. Flight altitudes were considerably higher during pre-breeding (median 388 m a.g.l.) than post-breeding migration (199 m a.g.l.), while no meaningful differences emerged in flight speed. Despite generally low flight altitudes, we observed large variability, with some individuals exceeding 1500 m a.g.l. (3% of locations) and one reaching 2429 m a.g.l. (2565 m a.s.l.). These findings indicate that Eurasian woodcocks migrate at relatively lower altitudes than other migratory landbirds, yet can perform high-altitude flights when required. Seasonal variation in flight altitude suggests that migrating woodcocks adaptively tune flight altitudes to maximize exploitation of prevailing westerlies (tailwinds) at higher altitudes during spring.