Quantum Magnets in Bird Eyes

Illustration adapted from cover graphic by Kyle Bean for Scientific American

The April issue of Scientific American has a fascinating piece of research into the mystery of how birds find their way in their migrations. It’s long been known that many species can fly thousands of miles from their breeding ground to their wintering places and back, and land in the same exact spot each time. Many birds manage this by traveling at night. How exactly they do it has been a baffling scientific puzzle.

It’s been established previously that birds can read stars to help them navigate, and that the smells of land and weather play a role. Some 50 years ago, researchers demonstrated that birds also navigate by the earth’s magnetic field. Birds held captive in a circular cage in an enclosed shed at the beginning of migration season would agitate in the direction of their migrating route. When the cage was shielded from magnetic fields, the birds moved in random directions. If the researchers installed an artificial magnetic field, they could manipulate the birds’ motion by altering the field’s direction. Clearly, the birds could sense the magnetic field and used the field to guide the direction of their migratory flight.

But how did the birds sense the earth’s magnetic field? The obvious theory was that somewhere in their brain or in their eyes or in between they had some iron particles that acted like our conventional compasses. Unfortunately, it wasn’t so easy. No iron or iron-like molecules in the birds’ anatomy could provide a convincing explanation of their magnetic sensitivity.

Now researchers have discovered a mechanism that promises to explain how birds do it. The explanation relies on a quantum-mechanical property of electrons known as “spin.” Particles with spin, even though they contain no iron or anything resembling it, have magnetic properties. When two of these particles are created at the same time, they form a “radical pair” that changes its polarity millions of times per second and, most importantly, is influenced by extremely subtle magnetic fields such as those of the earth. After decades of experiments, scientists were able to show that birds’ retinas contain a protein, cryptochrome, that generates these “radical pairs” when struck by a photon of blue light. The earth’s magnetic field causes the oscillation between the members of the pair to shift in one direction or another depending on its orientation. That shift generates a chain of chemical reactions that ends as a signal in the neurotransmitter pathways of the bird’s brain. Combined with other information, the birds find their way.

The article details the stages in the years of research done to develop and solidify this explanation. Most of it is beyond my level of physics, but the authors do a good job explaining the matter to the lay person. The authors also modestly admit that this research does not solve every question relating to birds’ navigation, and that more needs to be done. I very much agree with their concluding paragraph:

When you next see a small songbird, pause for a moment to consider that it might recently have flown thousands of kilometers, navigating with great skill using a brain weighing no more than a gram. The fact that quantum spin dynamics may have played a crucial part in its journey only compounds the awe and wonder with which we should regard these extraordinary creatures.

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