A compass made of light
The ability of migratory birds, like the European Robin (Erithacus rubecula), to navigate thousands of kilometers is partly explained by a biological compass. Research that began in the 1960s at Goethe University Frankfurt by zoologists Wolfgang and Roswitha Wiltschko demonstrated that birds possess an internal magnetic sense. Their experiments, which involved placing robins in cages where the magnetic field could be artificially manipulated with Helmholtz coils, showed the birds would orient themselves according to the field. The the avian compass is an "inclination compass." It doesn't detect the polarity (north vs. south) but rather the angle of the Earth's magnetic field lines relative to the ground. Birds perceive "poleward," where the field lines dip downwards, and "equatorward," where they point upwards.
This sense is also light-dependent; it functions only when the bird is exposed to short-wavelength light, from UV to blue or green. Under yellow or red light, the birds' ability to orient themselves vanishes. This light dependency pointed away from simple iron-based magnetoreception and toward a more complex photochemical process, leading scientists to investigate the bird's eye as the location of the sensor. The leading theory is that birds "see" the magnetic field as a visual pattern superimposed on their normal vision, a pattern that changes as they turn their heads.
The radical pair mechanism
The current explanation for this avian sense is the radical pair mechanism, a quantum mechanical process. The candidate molecule for this sensor is a protein in the retina called Cryptochrome 4 (Cry4). When a photon of blue light strikes the Cry4 molecule, it causes an electron to jump from one part of the protein to another. This creates two molecules that each have an unpaired electron, known as a radical pair. The electron spins of this pair are quantumly entangled, meaning their states are linked.
The Earth's magnetic field, which is very weak at just 25 to 65 microteslas, is still strong enough to influence the spin state of these electrons. The field affects how long the radical pair remains in its entangled "singlet" state versus an unlinked "triplet" state. This difference in spin state alters the chemical products that the cryptochrome reaction produces. This chemical signal is then thought to be converted into a nerve impulse, creating a visual pattern that provides the bird with directional information. Quantum calculations show that four specific amino acids within the 527 that make up the Cry4 protein, a chain of tryptophans, allow this magnetically sensitive electron transfer.