The discovery of biogenic magnetite in the radula teeth of chitons (Lowenstam, 1962) and in bacteria (Blakemore, 1975) led to speculation about magnetite (Fe3O4) crystals being involved in magnetoreception. Involvement of these crystals

chitons (Lowenstam, 1962) and in bacteria (Blakemore, 1975) led to speculation about magnetite (Fe3O4) crystals being involved in magnetoreception. Involvement of these crystals became more likely when magnetite was also found in animal species that were known to orient with the help of the magnetic field (e.g. Gould et al., 1978; Walcott et al., 1979) (for a review, see Kirschvink et al., 1985). Various types of possible receptor using magnetite have been described on the basis of different functional principles, some of which involve single domains and others, superparamagnetic particles (e.g. Yorke, 1979; Kirschvink and Gould, 1981; Kirschvink, 1989; Shcherbakov and Winklhofer, 1999). Several authors tried to verify the role of magnetite-based receptors in orientation processes by behavioral experiments. The first attempts produced inconclusive results (e.g. Gould et al., 1980; Walcott et al., 1988); later ones were more promising (Kirschvink and Kobayashi-Kirschvink, 1991; Kirschvink et al., 1997). Birds have always been of particular interest for the study of magnetoreception because they rely strongly on the Earth’s magnetic field for orientation and navigation. They appear to use information from the geomagnetic field in two ways, namely (1) as a compass for direction finding and (2) as part of their navigational ‘map’ for determining positions (for a review, see R. Wiltschko and Wiltschko, 1995). These probably involve separate receptor systems with different types of receptor cells, as the biophysical constraints on them differ markedly (e.g. Kirschvink and Walker, 1985). In birds, magnetite was found in the head, particularly in the ethmoid region above the beak (Walcott et al., 1979; Beason and Nichols, 1984; Beason and Brennan, 1986; Kirschvink and Walker, 1986; Edwards et al., 1992) and in the cutis of the upper mandible (Hanzlik et al., 2000; Winklhofer et al., 2001). These parts of the head are innervated by the ophthalmic nerve, a branch of the nervus trigeminus; electrophysiological recordings from this nerve and from the trigeminal ganglion of Bobolinks Dolichonyx oryzivorus (Icteridae) revealed units that responded to changes in the intensity of the magnetic field 3031 The Journal of Experimental Biology 205, 3031–3037 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4431