Abstract
As long as bird migrants are not pushed upwards by topographical features or guided to higher altitudes by increasing wind assistance, less than 10 % of migrants fly on average higher than 2000 m above ground level (a.g.l.), and less than 5 % higher than 3000 m. Automatic radar-tracking of migrants at the Baltic Sea and in the Sahara allows visualizing typical examples of diurnal variation in migratory intensity and vertical distribution. For various stations distributed from Europe to the trade-wind zone, we calculated average ground speeds (øVg) and air speeds (øVa) over all altitudes and compared these with those of the birds ≥ 3000 m above sea level (a.s.l.). In autumn, the proportion of birds above 3000 m a.s.l. was not substantially higher than 3 % in the European lowlands, on the Balearic Islands and the Negev-Highlands; only in the western Sahara, the proportion reached 4.7 %. In spring, the proportion of high flying birds was 5.4 % on the Balearic Islands, 6.6 % in the Negev, and 21 % in the Sahara. At all stations, the average ground speed (øVg) of high flying birds was significantly higher than their øVa and øVg over all heights. øVg of the high flying birds was 13.6 m/s above the lowlands of central Europe, 14.7 m/s above the Alps, and about 18 m/s above the Mediterranean; in the trade-wind zone it was ~17 m/s in autumn and more than 21 m/s in spring. Although most high flying autumn migrants are confronted with opposing winds in Europe and in the western Sahara, a majority of them found windows with wind support, indicating efficient selectivity. This selectivity is similar for migrants above the western Mediterranean and in the trade-wind zone in spite of differing atmospheric conditions. Pronounced selection for wind support in spring suggests time minimization on return flights towards the breeding grounds.
