The relevance of wind for optimal migration theory

Tracking radar measurements of winds at the Eight levels of bird migration during several migratory seasons in central Europe, southern Israel and the western Mediterranean are analysed with regard to optimal migratory behaviour in relation to wind. Birds migrating through central Europe and the Mediterranean area have to cope with windspeeds commonly ranging from 50 to 100% of their normal airspeed. Opposing winds prevail in central Europe. while at the western and eastern edge of the Mediterranean head- and tailwinds occur in similar frequencies. Winds are highly variable in time and altitude. A bird migrating selectively during nights with favourable wind conditions speeds up its flight by 30%, (on average) compared to an individual disregarding the wind situation. Selecting the most profitable Eight altitude may result in an additional gain of 40% in Eight speed. Therefore, taking into account the wind situation carefully, a bird can almost double its Eight speed and save about half of the energy required fbr its migratory journey through central Europe and the Mediterranean. The time needed for refuelling decreases accordingly or the safety margins provided by fat reserves can be increased. For birds flying long distances in one step, the selection of favourable winds should be more important than adjusting departure to local fat accumulation rates.