Effect of Wavelength of Light and Pulse Magnetisation on Different Magnetoreception Systems in a Migratory Bird

Abstract

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 magnetoreception is based on biogenic magnetite. Both mechanisms are supported by experimental evidence, but whether the information they provide involves the magnetic compass or the ‘map’ is still open. In order to identify the relevance of light-dependent or magnetite-transduced processes in magnetoreception, juvenile migratory birds were tested for their orientation behaviour in the natural geomagnetic field as the only directional cue available to them. The test birds were juvenile Tasmanian silvereyes (Zosterops l. lateralis), which were caught on their 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 migratory direction, while they were disoriented under red light (633 nm). This coincides with previous findings on adult silvereyes and suggests that light-dependent processes are involved in an orientation mechanism used by both juvenile and adult migrants, namely the magnetic compass. (2) A short, high-intensity magnetic pulse, a treatment designed to alter the magnetisation of magnetite, did not affect the young birds´ orientation. They continued to select their seasonally appropriate migratory direction. In contrast, adult silvereyes from the same population had responded in a previous study with a 90° clockwise deflection from their normal migratory course. These results suggest that (a) magnetite is involved in an orientation mechanism used exclusively by adult migrants; and (b) a magnetite-based receptor is associated with the navigational ‘map’, which provides information on geographic position.

Evidence for a navigational map stretching across the continental U.S. in a migratory songbird

“…A navigation system presumed to be based on experience has also been shown in caged adult migrants (27,28). However, previous experiments that tested migratory orientation in cages suggested that juveniles are able to compensate for the displacement.…”

Bird navigation: what type of information does the magnetite-based receptor provide?

“…In previous studies, an effect of the pulse was not observed in young, inexperienced migrants (Munro et al 1997) and it was suppressed in birds, whose ophthalmic nerve had been anaesthetized (Beason & Semm 1996). In both the cases, the birds continued in their normal Table 1.…”

“…Details of these results allowed an interpretation of these findings: the pulse effect was restricted to old, experienced birds, thus indicating that a learned system was involved, whereas the disorientation under long-wavelength light was observed in old, experienced and young, inexperienced migrants alike, thus suggesting an effect on an innate system (Munro et al 1997). Hence, we tentatively concluded that the magnetite-based mechanism provided birds with magnetic information that was interpreted with the help of the experience-based navigational 'map' to indicate position, while radical pair processes provided directional information for the innate magnetic compass (Munro et al 1997;see Wiltschko & Wiltschko 2005;Wiltschko & Wiltschko 2006 for review).…”

“…(b) Pulse treatment and data collection The test protocol replicates that of the previous studies ( Wiltschko et al 1994( Wiltschko et al , 1998Munro et al 1997). Testing began with a series of six control tests in the local geomagnetic field (mNZ3608, 56 000 nT, K628 inclination) to determine the directional preference of each individual in order to assure that the birds showed appropriate migratory orientation.…”

“…Hence, we tentatively concluded that the magnetite-based mechanism provided birds with magnetic information that was interpreted with the help of the experience-based navigational 'map' to indicate position, while radical pair processes provided directional information for the innate magnetic compass (Munro et al 1997;see Wiltschko & Wiltschko 2005;Wiltschko & Wiltschko 2006 for review). This was in agreement with electrophysiological studies, where units in the trigeminal system (ophthalmic nerve, trigeminal ganglion) that innervate the region, where magnetite is located in birds (e.g.…”