EFFECT OF WAVELENGTH OF LIGHT AND PULSE MAGNETIZATION ON DIFFERENT MAGNETORECEPTION SYSTEMS IN A MIGRATORY BIRD

Two hypotheses on magnetoreception in animals are currently discussed. The first hypothesis is based on light-dependent processes associated with the visual system, while the second hypothesis suggests that mag netoreception is based on biogenic magnetite. Both mechanisms are supp orted by experimental evidence, but whether the information they provi de involves the magnetic compass or the 'map' is still open. In order to identify the relevance of light-dependent or magnetite-transduced p rocesses in magnetoreception, juvenile migratory birds were tested for their orientation behaviour in the natural geomagnetic field as the o nly directional cue available to them. The test birds were juvenile Ta smanian silvereyes (Zosterops l. lateralis), which were caught on thei r native island soon after fledging, before they had an opportunity to establish a navigational 'map'. (1) Under 'white' (full spectrum) and green light (571 nm), they were well oriented in their appropriate mi gratory direction, while they were disoriented under red light (633 nm ). This coincides with previous findings on adult silvereyes and sugge sts that light-dependent processes are involved in an orientation mech anism used by both juvenile and adult migrants, namely the magnetic co mpass. (2) A short, high-intensity magnetic pulse, a treatment designe d to alter the magnetisation of magnetite, did not affect the young bi rds' orientation. They continued to select their seasonally appropriat e migratory direction. In contrast, adult silvereyes from the same pop ulation had responded in a previous study with a 90 degrees clockwise deflection from their normal migratory course. These results suggest t hat (a) magnetite is involved in an orientation mechanism used exclusi vely by adult migrants; and (b) a magnetite-based receptor is associat ed with the navigational 'map', which provides information on geograph ic position.