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Billions of songbirds breeding in the Western Palaearctic cross the largest desert of the world, the Sahara, twice a year. While crossing Europe, the vast majority use an intermittent flight strategy, i.e. fly at night and rest or feed during the day. However, it was long assumed that they overcome the Sahara in a 40h non-stop flight. In this study, we observed bird migration with radar in the plain sand desert of the Western Sahara (Mauritania) during autumn and spring migration and revealed a clear prevalence of intermittent migration. Massive departures of songbirds just after sunset independent of site and season suggests strongly that songbirds spent the day in the plain desert. Thus, most songbirds cross the Sahara predominately by the intermittent flight strategy. Autumn migration took place mainly at low altitudes with high temperatures, its density decreased abruptly before sunrise, followed by very little daytime migration. Migration was highly restricted to night-time and matched perfectly the intermittent flight strategy. However, in spring, when migratory flights occurred at much higher altitudes than in autumn, in cool air, about 17% of the songbird migration occurred during the day. This suggests that flying in high temperatures and turbulent air, as is the case in autumn, may lead to an increase in water and/or energy loss and may prevent songbirds from prolonged flights into the day. [ABSTRACT FROM AUTHOR]