A limitation of standardized mist netting for monitoring migration is caused by the lack of knowledge about the relationship between trapped birds and birds flying aloft. Earlier studies related nocturnal radar counts with trapping data of the following day. In this study, we compared for the first time data gathered simultaneously by radar and mist netting, separately for diurnal and nocturnal migration. Trapping numbers were strongly correlated with migratory intensities measured by radar (r>0.6). A multiple regression analysis, including wind speed and wind direction explained 61% of variation in the number of captures. During the night, and particularly with favourable winds, birds flew at higher altitudes and hence escaped the nets to a higher proportion. The number of nocturnal migrants trapped during daytime was well correlated with migratory intensities observed by radar in the preceding night. The diurnal time patterns, however, revealed fundamental differences between trapping counts and radar observations. This was mainly due to increasing and decreasing flight altitudes in the course of the night, and by the limitations of the radar technique that underestimates migratory intensities during the day when birds aggregate in flocks. In relation to the migratory intensity recorded by radar, diurnal migrants are trapped in a much higher proportion than nocturnal migrants. Finally, our results confirm that trapping data from a site hardly used for stopover are well suited to represent the ongoing migration during the day and night.
SummaryNocturnal autumn bird migration was studied for the first time along the southern edge of the western Sahara. A passive infrared device provided data on the intensity and direction of nocturnal passage at 9 different sites for 20 nights from dawn to dusk. The results support the idea that many bird migrants use an intermittent flight strategy when crossing the Sahara. Flight directions along the southern border of the desert were SSW, similar to those known from southern Spain, and thus, did not confirm the expected shift towards SE before the desert crossing, as derived from former laboratory experiments.
Nocturnal autumn bird migration was studied for the first time along the southern edge of the western Sahara. A passive infrared device provided data on the intensity and direction of nocturnal passage at 9 different sites for 20 nights from dawn to dusk. The results support the idea that many bird migrants use an intermittent flight strategy when crossing the Sahara. Flight directions along the southern border of the desert were SSW, similar to those known from southern Spain, and thus, did not confirm the expected shift towards SE before the desert crossing, as derived from former laboratory experiments.
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