Migratory birds are remarkably proficient navigators. Flying alone and often over great distances,
they use various directional cues including crucially, a light-dependent magnetic compass.
Some birds can sense Earth’s magnetic field, using it to navigate. But precisely how they do this has long
remained a mystery. Now researchers have confirmed that a protein found in birds’ eyes displays a quantum
mechanical phenomenon that makes it sensitive to magnetic fields. The researchers suggest this mechanism could be behind a bird’s magneto-reception abilities. Only two basic mechanisms are considered theoretically viable in terrestrial animals: iron-mineral based magnetoreception and radical-pair based magnetoreception. We are discussing the radical pair mechanism of compass magneto-reception in birds.
It is very challenging to sense magnetic field as weak as that of the Earth’s using only biologically
available materials. Cry4 cryptochrome is a protein clustered in the retina of bird’s eye, especially sensitive to blue light. This protein is made of a kind of molecule that sometimes has an odd number of electrons.
The scientists theorize that as incoming light enters the eye, a photon hits the cryptochrome proteins, exciting the electron on it. That energy is transferred between two molecules within the protein yielding a radical pair, two free electrons that are quantum entangled and correlated. The magnetic spins of this can take on two states either as singlets with antiparallel spin or as triplets with parallel spin. The pair swings between these two states. This change in the balance of the lifetimes of the triplet state relative to the singlet state is thought to be the basis of magnetoreception. This coherent quantum state can reacts to a weak external magnetic field like Earth’s magnetic field.
As birds turn or move, the electron spins differently causing different messages to be sent to the
bird’s brain, regarding the direction of the magnetic field. It is transmitted by the optic nerve to the brain, where parts of the visual system seem to process the respective information.
Magnetic intensity is perceived by magnetite-based receptors in the beak region; the information is transmitted by the ophthalmic branch of the trigeminal nerve to the trigeminal ganglion and the trigeminal
brainstem nuclei.
Yet in spite of considerable progress in recent years, many details are still unclear, among the details of the radical pair processes and their transformation into a nervous signal, the precise location of the magnetite based receptors and the centres in the brain where magnetic information is combined with other
navigational information for the navigational processes.
References:
Also have a look at “Do magnets make your blood clot?“
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