British and American scientists have built a chemical model that places some credence to the radical chemistry hypothesis of magnetoreception to explain how migratory birds use Earth's magnetic field as a compass. The hypothesis argues that incident light on photoreceptor proteins in a bird's retina could create radical pairs. These short-lived reaction intermediates' fate, whether returning to a resting state or reacting to give a different radical product, could be modulated by Earth's magnetic field and could act as a magnetic field sensor or compass for birds. Peter J. Hore at Oxford University and his colleagues have build a carotenoid-porphyrin-fullerene model system that produces radical electron pairs that are measurably affected by Earth's magnetic field. Earth's magnetic field has many measurable components, such as direction, intensity, and inclination. The different mechanisms of magnetoreception could provide complementary sorts of information to the migrating birds.
Everts, S. (2008). Guiding migration.