Redstarts, Phoenicurus phoenicurus, can orient in a true-zero magnetic field

I tested the migratory orientation of redstarts in a true-zero magnetic field to elucidate the importance to this species of access to either magnetic or celestial cues. I also tested the validity of the assumption on which all funnel experiments are based: that what we observe in an orientation funnel reflects what the bird would do if actually migrating. In a set of funnel experiments, I tested 47 night-migrating redstarts caught during their first autumn migration. Each bird was tested once under each of four experimental conditions in a semi-randomized block-design. Upon completion of the final funnel tests the birds were fitted with light indicators and released. The results showed that redstarts can find the migratory direction on the basis of access to either celestial or magnetic cues. Thus, when celestial cues were available they could orient in a true-zero magnetic field. In addition, a starry sky facilitated high migratory activity as well as a clearly directed orientation. The release experiments showed that the vanishing bearings on nights with no wind were in good agreement with the direction of the funnel activity, but that the vanishing bearings were strongly influenced even by light wind. (C) 1998 The Association for the Study of Animal Behaviour.