High-resolution telemetry reveals a globally important migration bottleneck for Lesser Spotted Eagles (Clanga pomarina) on the shores of Lake Tanganyika

Migratory routes align with leading lines where favorable soaring conditions occur. Soaring raptors, including the Lesser Spotted Eagle ( Clanga pomarina ), avoid large water bodies and mountain ranges where conditions are less conductive to sustained soaring flight, resulting in mass migrations through migratory bottlenecks. Lesser Spotted Eagles breed in Central and Eastern Europe and parts of Asia, overwintering in southern Africa. Nearly the entire global population follows a narrow 5360 km flyway from the Middle East to southern Africa. Since 1994, 42 adult eagles were tracked using a range of telemetry technologies. Here, we leverage these tracking data and reveal that Lake Tanganyika forms a major migratory barrier for the species. All tagged eagles passed through a narrow, c. 8-km-wide bottleneck at the lake’s southern tip, near the Tanzania-Zambia border. This route adds a detour of up to 500 km, particularly for individuals wintering in Angola. High-resolution GPS-GSM telemetry enabled the visualisation of the routes navigated by four adult eagles between 2012 and 2016 as they passed through the narrow migratory bottleneck. On their return northward migration, eagles pass Lake Tanganyika over a much broader front. This study provides the first documentation of the “Lake Tanganyika Bottleneck” for this species. The bottleneck’s existence is likely driven by a combination of topography, habitat, and climatic conditions. Given our findings, we call for regional ornithologists to conduct standardized migration counts at the site to assess its significance for other migratory soaring birds. Migratory bottlenecks offer a vital natural stage for public engagement activities and capacity building for conservation initiatives within the region.