  Experiments on the effect of radio-frequency (RF) magnetic fields on the
magnetic compass orientation of migratory birds are analyzed using the theory
of magnetic resonance. The results of these experiments were earlier
interpreted within the radical-pair model of magnetoreception. However, the
consistent analysis shows that the amplitudes of the RF fields used are far too
small to noticeably influence electron spins in organic radicals. Other
possible agents that could mediate the birds' response to the RF fields are
discussed, but apparently no known physical system can be responsible for this
effect.
