Orientation in pied flycatchers: the relative importance of magnetic and visual information at dusk

We investigated the orientation of juvenile pied flycatchers, Ficedula hypo leuca, during autumn migration in south Sweden using orientation cage exper iments, to study the relative importance of visual and magnetic information at sunset. We performed cage tests under 12 experimental conditions that m anipulated the geomagnetic and visual sunset cues available, for orientatio n: natural clear skies in the local or a vertical magnetic field; simulated total overcast in the local pr a vertical magnetic field; natural pattern of skylight polarization and directional information from stars screened of f, with the sun's position as normal or shifted 120 degrees anticlockwise w ith mirrors; reduced polarization in the local or a vertical magnetic field ; directions of polarization (e-vector) NE/SW and NW/SE, respectively, in t he local or a vertical magnetic field. The pied flycatchers were significan tly oriented towards slightly south of west when they could use a combinati on of skylight and,geomagnetic cues. The mean orientation was significantly shifted along with the deflection of the sunset position by mirrors. Reduc ed polarization had no significant effect on orientation either in the loca l, or in a vertical, magnetic field. The birds tended to orient parallel wi th the axis of polarization, but only when the artificial e-vector was alig ned NW/SE. The mean orientation under simulated total overcast in a vertica l, and in the local, magnetic field was not significantly different from ra ndom. It is difficult to rank either cue as dominant over: the other and we conclude that both visual and magnetic cues seem to be important for the b irds' orientation when caught and tested during active migration. (C) 1999 The Association for the Study of Animal Behaviour.