with unpaired electrons, at a suitable distance from each other. When these can exist in states either with their spin axes in the same direction, or in opposite directions, the molecules oscillate rapidly between the two states. That oscillation is extremely sensitive to magnetic fields. Because the Earth's magnetic field is extremely weak, at 0.5 gauss, the radical pair mechanism is currently the only credible way that the Earth's magnetic field could cause chemical changes. In 1978, Schulten and colleagues proposed that this was the mechanism of magnetoreception. In 2000, scientists proposed that cryptochrome – a flavoprotein in the rod cells in the eyes of birds – was the "magnetic molecule" behind this effect. It is the only protein known to form photoinduced radical-pairs in animals. The function of cryptochrome varies by species, but its mechanism is always the same: exposure to blue light excites an electron in a chromophore, which causes the formation of a radical-pair whose electrons are quantum entangled, enabling the precision needed for magnetoreception. Many lines of evidence point to cryptochrome and radical pairs as the mechanism of magnetoreception in birds: Despite 20 years of searching, no biomolecule other than cryptochrome has been identified capable of supporting radical pairs. In cryptochrome, a yellow molecule flavin adenine dinucleotide (FAD) can absorb a photon of blue light, putting the cryptochrome into an activated state: an electron is transferred from a tryptophan amino acid to the FAD molecule, forming a radical pair. Of the six types of cryptochrome in birds, cryptochrome-4a (Cry4a) binds FAD much more tightly than the rest. Cry4a levels in migratory birds, which rely on navigation for their survival, are highest during the spring and autumn migration periods, when navigation is most critical. The Cry4a protein from the European robin, a migratory bird, is much more sensitive to magnetic fields than similar but not identical Cry4a from pigeons and chickens, which are non-migratory. These findings together suggest that the Cry4a of migratory birds has been selected for its magnetic sensitivity. Behavioral experiments on migratory birds also support this theory. Caged migratory birds such as robins display migratory restlessness, known by ethologists as Zugunruhe, in spring and autumn: they often orient themselves in the direction in which they would migrate. In 2004, Thorsten Ritz showed that a weak radio-frequency electromagnetic field, chosen to be at the same frequency as the singlet-triplet oscillation of cryptochrome radical pairs, effectively