The role of wind in passerine autumn migration between Europe and Africa

Large ecological barriers such as oceans and deserts have considerablyshaped the migratory strategies of birds. The ecological barriersposed by the Alps, the Mediterranean Sea, and the Sahara seemto prevent most long-distance migrants from flying on a directsouthward course from Europe to Africa. Migratory routes towardsouthwest and southeast prevail. These two flyways differ withrespect to topography, refueling possibilities, and wind conditions.Aiming at a better understanding of the evolution of both flywaysin spite of differing conditions, we studied potential survivalof passerine birds on their first autumn migration from northernEurope to tropical Africa by means of a computer simulation.Considering real wind conditions at 850 mb (approximately 1500m above sea level), the survival rates of birds with southeasterly(SE) migratory directions were much higher than those of birdswith southwesterly (SW) directions. With the possibility tochoose the altitude (from four levels) with the most favorablewind, both SE and SW migrants had similar high survival, butonly with refueling opportunities in northwest (NW) Africa forSW migrants. Our results suggest that the southwestern flywaydepends on the selection of days, but especially altitudes,with favorable wind conditions and on refueling opportunitiesin NW Africa. The SE flyway is privileged by the frequent favorablewind conditions for crossing the eastern Mediterranean Sea andthe Egyptian desert, where refueling sites are almost absent.Both autumn migration routes would be unlikely without windassistance.