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

Large ecological barriers such as oceans and deserts have considerably shaped the migratory strategies of birds. The ecological barriers posed by the Alps, the Mediterranean Sea, and the Sahara seem to prevent most long-distance migrants from flying on a direct southward course from Europe to Africa. Migratory routes toward southwest and southeast prevail. These two flyways differ with respect to topography, refueling possibilities, and wind conditions. Aiming at a better understanding of the evolution of both flyways in spite of differing conditions, we studied potential survival of passerine birds on their first autumn migration from northern Europe to tropical Africa by means of a computer simulation. Considering real wind conditions at 850 mb (approximately 1500 m above sea level), the survival rates of birds with southeasterly (SE) migratory directions were much higher than those of birds with southwesterly (SW) directions. With the possibility to choose the altitude (from four levels) with the most favorable wind, both SE and SW migrants had similar high survival, but only with refueling opportunities in northwest (NW) Africa for SW migrants. Our results suggest that the southwestern flyway depends on the selection of days, but especially altitudes, with favorable wind conditions and on refueling opportunities in NW Africa. The SE flyway is privileged by the frequent favorable wind conditions for crossing the eastern Mediterranean Sea and the Egyptian desert, where refueling sites are almost absent. Both autumn migration routes would be unlikely without wind assistance. Copyright 2005.