Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination.
It is known that night-migratory songbirds use a magnetic compass measuring the magnetic inclination angle, i.e. the angle between the Earth's surface and the magnetic field lines, but how do such birds orient at the magnetic equator? A previous study reported that birds are completely randomly oriented in a horizontal north-south magnetic field with 0° inclination angle. This seems counter-intuitive, because birds using an inclination compass should be able to separate the north-south axis from the east-west axis, so that bimodal orientation might be expected in a horizontal field. Furthermore, little is known about how shallow inclination angles migratory birds can still use for orientation. In this study, we tested the magnetic compass orientation of night-migratory Eurasian blackcaps (Sylvia atricapilla) in magnetic fields with 5° and 0° inclination. At 5° inclination, the birds oriented as well as they did in the normal 67° inclined field in Oldenburg. In contrast, they were completely randomly oriented in the horizontal field, showing no sign of bimodality. Our results indicate that the inclination limit for the magnetic compass of the blackcap is below 5° and that these birds indeed seem completely unable to use their magnetic compass for orientation in a horizontal magnetic field.
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Susanne
S
Schwarze
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Friederike
F
Steenken
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Nadine
N
Thiele
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Dmitry
D
Kobylkov
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Nele
N
Lefeldt
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
David
D
Dreyer
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Nils-Lasse
NL
Schneider
AG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany.|Research Center Neurosensory Science, University of Oldenburg, D-26111 Oldenburg, Germany.
Henrik
H
Mouritsen
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