ORIENTATION IN RELATION TO EXPOSURE TO THE SETTING SUN IN SOME PASSERINE TRANS-SAHARAN MIGRANTS

Four species of nocturnal trans-saharan passerine migrants (Ficedula h ypoleuca, Phylloscopus trochilus, Phoenicurus phoenicurus, Sylvia bori n) were tested in Emlen funnels to assess the relative importance of t he setting sun and stars in determining the nocturnal course. The expe riments took place during spring migration at a Mediterranean stop-ove r site (Sardinia, Italy). The birds were caught during the day in mist -nets and kept in opaque plastic cages preventing them seeing outside, until the time of the experiment, on the same night; each bird was te sted only once. Two experimental groups were exposed to the sunset, th e first including the sight of the sun disc, the other after the sun h ad set. A third group was kept in the closed cages until the time of t he test. A fourth group was directly tested in the funnels at sunset t ime, after the sun disc had set. The first three groups were tested in the funnels later, in complete darkness. Exposure and tests occurred under the same sky conditions, either clear or overcast. Under a clear sky, 10 out of 16 distributions are oriented in the expected migrator y direction, or have a tendency towards it, irrespective of their expo sure to the setting sun. This shows that the cues present at sunset ar e not necessary for the orientation of these birds, whose ability to o rient correctly in the absence of any celestial cues is also confirmed by the results of the tests performed in overcast conditions with no previous exposure to Visual cues. Unexpectedly, exposure to the settin g sun with an overcast sky, followed by a test in the same overcast co nditions lead to disorientation, suggesting that exposure to visual cu es makes the birds look, at least at first, for other similar cues bef ore turning to non-visual compass information.