Orientation of night-migrating passerines kept and tested in an inverted magnetic field

From 1992 to 1999 more than 500 funnel experiments were carried out with robins, redstarts, whitethroats, and pied flycatchers. The purpose was to elucidate the orientation of birds caged and tested for a prolonged time in an inverted magnetic field, i.e. a field where the vertical component was reversed (pointed upwards). Contrary to expectations, the results did not suggest that orientation was governed by a magnetic inclination compass; generally no differences were found between birds tested in the inverted field and those tested in the natural one. However, occasional patterns of (bimodal) orientation at right angles appeared, probably elicited by the inverted field, and contradictions between stellar and magnetic information. Possibly, the significance and kind of magnetic orientation in passerine birds is less well understood than is normally supposed.