Extremely detoured migration in an inexperienced bird: interplay of transport costs and social interactions

Abstract

We tagged two juvenile short-toed eagles in southern Italian peninsula with GPS satellite transmitters. According to previous visual observations, two different migratory routes for Italian short-toed eagles to reach Africa in autumn have been proposed: via Sicily and via Gibraltar. These routes include different over-water distances to cross the Mediterranean Sea, and thus different proportions of flight modes (soaring-gliding vs flapping-gliding) with resulting different transport costs. Considering different scenarios of energy cost of transport, with flapping-gliding flight over water being more costly than flying over land using soaring-gliding flight, we predicted a maximum optimal detour of 1218 km. Both individuals reached Africa using the longest, detoured, route, avoiding the longest water crossing. To achieve this they began migrating northwards, keeping for ca 700 km a direction opposite to that followed by any other migrating bird from the Northern hemisphere in autumn. The comparison of optimal detour predictions with observed migratory tracks suggests that this migratory strategy prioritizes not only energy minimization, but also safety, given the mortality risk associated with the sea crossing. Finally, it is unlike that these inexperienced individuals followed such a complex route relying only on endogenous information and we therefore suggest, also on the basis of field observations, that social interactions (adult guidance) allow these individuals to learn the detoured route.