Liechti, F. & H. Schmaljohann
(* = Kurzbeitrag)
Vogelzug über der westlichen Sahara.
(von 1994 bis 2006 vergeben)
Vogelzug, Frühlingszug, Herbstzug, Zugverhalten, Zugroute, Zughöhe, Rastplatz, Sonnenuntergang, Wind, Temperatur, Energieverbrauch, Wasserverlust, Singvogeldichte, Wüste
Songbird migration across the western Sahara. Billions of songbirds cross the largest desert of the world, the Sahara, twice a year. The vast majority of these trans-Sahara migrants use an intermittent migration strategy when crossing Europe; i.e. they fly at night and rest or feed during the day. In the Sahara songbirds encounter temperatures up to 50 °C, no water and only a few oases. These inhospitable ground conditions have raised the hypothesis that songbirds overcome the Sahara in a 40–60 hour non-stop flight. In this study, we investigated bird migration across the western Sahara in Mauritania at an oasis and two bare desert sites in autumn and spring. We quantified for the first time the temporal and spatial pattern of songbird migration across the Sahara. After sunset, songbird densities increased considerably at all sites and during both seasons. In the course of the night songbird densities remained relatively stable, but decreased after sunrise. Thus, songbirds migrate predominately at night and rest during the day. Under favourable wind conditions songbirds regularly prolonged their migratory flights into the day. This behaviour was especially pronounced in spring, when songbirds migrated at high altitude, where they flew in cool air and with tailwinds. In autumn 50 % of the songbird migration concentrated within the lowest 1000 m above ground, where favourable tailwinds prevailed, but where they had to face very warm temperatures (30 °C) and very dry conditions (humidity <20 %). We hypothesize that these harsh atmospheric conditions in autumn prevent songbirds to prolong their nocturnal flights into the day where conditions with respect of water loss are even worse than during the night. In spring songbirds flew at high altitude in cool air and could continue at these heights also into the day. However, we could only observe prolongations of nocturnal flights into the day on days with extra-
ordinary tailwinds, and also during these days songbird migration ceased in the course of the afternoon. We conclude that the increase in energy and/or water consumption from a nocturnal to a diurnal flight is more costly than resting on the ground in the bare sand desert, in spite of the inhospitable conditions.
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