Certain migratory birds can sense the earths magnetic field. The nature of this process is not yet properly understood. Here we offer a simple explanation according to which birds literally `see the local magnetic field: Our model relates the well-established radical pair hypothesis to the phenomenon of Haidingers brush, a capacity to see the polarisation of light. This new picture explains recent surprising experimental data indicating long lifetimes for the radical pair. Moreover there is a clear evolutionary path toward this field sensing mechanism: it is an enhancement of a weak effect that may be present in many species.
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