A variety of animal species is sensitive to changes in natural and artificial magnetic fields. The receptor mechanism for this ability has been described for a few species, most notably magnetotactic bacteria1 and potential receptors have been reported for such animals as honey bees, homing pigeons and dolphins2-5. Some species of migratory birds also perceive changes in magnetic field6. We show here that the bobolink (Dolichonyx oryzivorus), which has the longest transequatorial migratory path of any New World land bird7, responds to changes in the Earth's magnetic field indicating that it uses the magnetic information as a primary orientation cue during its migration. We suggest that the ability of the bobolink to detect magnetic fields is associated with deposits of iron oxide (probably magnetite) that lie in sheaths of tissues around the olfactory nerve and bulb and between the eyes, and also in bristles which project into the nasal cavity.
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