Migratory Orientation of Passerines at Dusk, Night and Dawn

Abstract

AbstractThe orientation of four different species of nocturnal passerine migrants was investigated in spring and autumn by orientation‐cage experiments at dusk, night, and dawn under natural clear skies. Our objective was to examine the orientation behaviour at different periods of a nocturnal migration flight, and to relate the orientation to visual orientation cues (the position of the sun). In robins, the mean orientation was W (autumn) and NW (spring) in the evening tests, N (both autumn and spring) in the middle of the night, and NE (both autumn and spring) at sunrise. The northerly mean orientation at night in both spring and autumn, agreed well with the expected migratory direction for robins (reoriented migration in autumn) as shown by ringing recoveries. A clockwise shift in mean orientation throughout a nocturnal migration flight period (spring and autumn) indicates that the birds may not compensate fully for the azimuthal change of either sun or stars at the horizon. Migratory activity was highest at dusk and dawn, with a reduction in activity in the middle of the night. Spring tests revealed a significantly higher incidence of migratory activity during the second h after sunset than in the autumn tests, indicating longer flights in spring when the birds are approaching the breeding areas. For pied flycatchers, redstarts and lesser whitethroats, the directional choices during spring were between NW and N in evening tests while the dawn experiments showed mean directions towards NE or E. A compromise orientation was observed during both spring and autumn between the expected migratory direction for the season and the position of the sun at both sunset and sunrise, resulting in orientation more to the north of the sun in spring and more to the south of the sun in autumn.