THE ALPS, AN OBSTACLE FOR NOCTURNAL BROAD FRONT MIGRATION - A SURVEY BASED ON MOON-WATCHING

With a new standardized moon-watching method nocturnal bird migration was studied over southern Germany, the area of the Alps and northern I taly in autumn 1994 and spring 1995. Simultaneous observations from mo re than 300 sites showed that in autumn broad front migration was devi ated and concentrated along the northern border of the Alps. Migration mas concentrated in the Swiss lowlands by a factor 2 to 3 compared to southern Germany Notable concentrations occurred also along the upper Rhine valley. Even under clear sky only about 20 to 30 % of the migra nts continued their flight over the mountain ridges towards Italy. Sou th of the Alps an important part of migration consisted of birds flyin g westwards, parallel to the border of the mountain ranges. Similar mi gratory intensify in autumn and in spring over northern Italy suggest concentrated spring migration south of the Alps. This idea is based on the fact that intensify of observed migration is lower to the north o f the Alps and in general the expected density of migration is lower i n spring (radar data and few moonmatching data). In principle, results showed that confronted with a mountain range (Vosges, Jura, Alps), mo st migrants prefer to deviate slightly from their innate migratory dir ection, instead of climbing up above the ridges, where they might enco unter unfavourable winds.