Magnetoreception and its trigeminal mediation in the homing pigeon

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Abstract
Two conflicting hypotheses compete to explain how a homing pigeon can return to its loft over great distances. One proposes the use of atmospheric odours1 and the other the Earth's magnetic field2,3,4 in the ‘map’ step of the ‘map and compass’ hypothesis of pigeon homing5. Although magnetic effects on pigeon orientation6,7 provide indirect evidence for a magnetic ‘map’, numerous conditioning experiments8 have failed to demonstrate reproducible responses to magnetic fields by pigeons. This has led to suggestions that homing pigeons and other birds have no useful sensitivity to the Earth's magnetic field9,10,11. Here we demonstrate that homing pigeons (Columba livia) can discriminate between the presence and absence of a magnetic anomaly in a conditioned choice experiment. This discrimination is impaired by attachment of a magnet to the cere, local anaesthesia of the upper beak area, and bilateral section of the ophthalmic branch of the trigeminal nerve, but not of the olfactory nerve. These results suggest that magnetoreception (probably magnetite-based) occurs in the upper beak area of the pigeon. Traditional methods of rendering pigeons anosmic might therefore cause simultaneous impairment of magnetoreception so that future orientation experiments will require independent evaluation of the pigeon's magnetic and olfactory systems.