1. School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
2. Department of Psychology, University of Auckland, Private Bag 92019, Auckland, New Zealand
3. Anatomy Department, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand
4. Present address: Department of Biology, Coker Hall, CB 3280, University of North Carolina, Chapel Hill, North Carolina 27599-3280, USA

Correspondence to: Cordula V. Mora^1,4 Email: cvmora@email.unc.edu

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 field^{2, 3, 4} in the 'map' step of the 'map and compass' hypothesis of pigeon homing⁵. Although magnetic effects on pigeon orientation^{6, 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 field^{9, 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.

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