Magnetite-based magnetoreception in birds: the effect of a biasing field and a pulse on migratory behavior

SUMMARY To test the hypothesis that single domain magnetite is involved in magnetoreception, we treated Australian silvereyes Zosterops l. lateralis with a strong, brief pulse designed to alter the magnetization of single domain particles. This pulse was administered in the presence of a 1 mT biasing field, either parallel to the direction of the biasing field (PAR group) or antiparallel (ANTI group). In the case of magnetoreceptors based on freely moving single domain particles, the PAR treatment should have little effect, whereas the ANTI treatment should cause remagnetization of the magnetite particles involved in a receptor and could produce a maximum change in that receptor's output for some receptor configurations. Migratory orientation was used as a criterion to assess the effect on the receptor. Before treatment, both groups preferred their normal northerly migratory direction. Exposure to the biasing field alone did not affect their behavior. Treatment with the pulse in the presence of the biasing field caused both the PAR and the ANTI birds to show an axial preference for the east—west axis, with no difference between the two groups. Although these results are in accordance with magnetite-based magnetoreceptors playing a role in migratory orientation, they do not support the hypothesis that single domains in polarity-sensitive receptors are free to move through all solid angles. Possible interpretations, including other arrangements of single domains and superparamagnetic crystals, are discussed.

Effect of a magnetic pulse on the orientation of silvereyes, zosterops l. lateralis, during spring migration

The orientation behaviour of Australian silvereyes, Zosterops l. lateralis, was tested during their spring migration, when they head southward to their Tasmanian breeding grounds. With only the local geomagnetic field as a cue, the birds significantly preferred their normal southerly migratory direction. Treatment with a short, strong magnetic pulse designed to alter the magnetization of single-domain magnetite led to a significant deflection towards the east for the next 4 days. This was followed by a period of non-oriented behaviour. From day 10 onwards, the birds returned to their original southerly headings. Together with previous findings, these data suggest that the navigational 'map' of these birds includes magnetic parameters and that a magnetite-based receptor provides them with information about their position. The transient nature of the effect is not easily explained on the basis of single-domain magnetite.

Effect of Wavelength of Light and Pulse Magnetisation 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 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.