ABSTRACT
The Montagu's harrier (Circus pvgargus) is a medium-sized long-distance migratory raptor with a southwest-Palearctic breeding distribution and an Afrotropical/Indomalayan wintering distribution. Their long and slender wings allow them to alternate between flapping-gliding and soaring-gliding flight. This makes Montagu's harriers less dependent on thermals compared to obligate soaring migrants, which has several consequences for their migration patterns. (1) Being a facultative soaring migrant allows Montagu's harriers to cross larger water bodies, resulting in broad front migration. Consequently, they are observed in comparatively small numbers at migration bottlenecks and watchpoints. (2) Furthermore, Montagu's harriers can use a relatively broad daily time window for travelling, and hence achieve high daily travel distances. Montagu's harriers, however, do not fly the whole day but frequently interrupt their flights for foraging and resting. Tailwinds have a large effect on daily travel distances not only because of the wind support itself but also because Montagu's harriers interrupt their flights less often, and continue to fly for a longer time period on days with tailwinds. Variation in weather accounts for almost all variation in travel distances among regions and between seasons.
A large satellite telemetry and GPS-tracking dataset is available for Montagu's harriers breeding in Europe. This revealed three main migration routes: a western route via Spain, a central route via Italy and an eastern route via Greece. Montagu's harriers generally follow the same route they used in autumn back in spring, although spring migration routes tend to lay (slightly) west of autumn routes. Only Eastern European breeding birds follow a clockwise loop by taking the eastern route via Greece in autumn and the central route via Italy in spring. These loop migration patterns are caused by dominating eastern winds in the Sahara Desert. Migration speeds are similar between routes but are lower in spring (114-162 km/d) compared to autumn (173-187 km/d) because of more and longer stopovers in north-west Africa during spring.
