Daytime passerine migrants over the Sahara — are these diurnal migrants or prolonged flights of nocturnal migrants?

Passerine migrants are usually divided into diurnal and nocturnal migrants. When crossing the Sahara it was expected that nocturnal migrants would continue their flight into the day. We investigated whether migration taking place in the Sahara during daytime comprises the normal diurnal migrant species or the nocturnal ones prolonging their flight into the day. Birds departing from the Sahel in spring had to cross an ecological barrier of 300km before reaching our study site, an oasis in central Mauritania. The intensity of passerine migration measured by radar varied from night to night and decreased towards sunrise. Under good wind conditions some passerine migration continued into the day. The landing tendency (sink rate) correlated negatively with the tail wind component. Transect counts on the ground revealed very low proportions of diurnal migrants, not matching the relatively high densities of passerine migration during the day, and a high correlation between transect density of nocturnal migrants and nocturnal passage of passerine migrants. Therefore we conclude that nocturnal passerine migrants are responsible for most of the recorded daytime passage (swifts, swallows and soaring birds being excluded). Flight and landing behaviour varied with environmental conditions and nocturnal passerine migrants adjusted their flight schedules opportunistically, continuing into the day in particularly favourable winds.