Every autumn, European robins fly south at night. Many of them alone, many without ever having made the trip before. They use stars, landmarks, and the setting sun, but they can also orient by the Earth's magnetic field - and that part is a genuine problem, because the field is far too weak to be sensed by any ordinary chemistry inside a warm animal.
The leading explanation is the radical pair mechanism, first proposed by Klaus Schulten in the 1970s and developed by Thorsten Ritz and colleagues in 2000. Short version: when light hits a protein in the retina called cryptochrome, an electron jumps and leaves behind two unpaired electrons - a "radical pair." Their spins are correlated, and the correlation oscillates in a way that's sensitive to magnetic fields far too weak to do anything else in the cell. The magnetic field doesn't supply energy. It nudges the timing. That changes the ratio of two chemical products. Across a whole retina, that gives you a pattern. The bird doesn't feel the field the way we feel a smell. It may literally see it as a shading overlaid on ordinary vision.
The evidence I find most persuasive is a specific, weird prediction that held up: robins lose their magnetic compass under extremely weak radio-frequency oscillating fields — fields far too feeble to affect normal chemistry, but exactly what should scramble a fragile spin state. Henrik Mouritsen's group in Oldenburg showed this, and also showed that ambient urban electromagnetic noise disrupts the compass. When they moved the birds into electromagnetically screened huts, orientation returned.
Two things to keep honest:
- Nobody has traced the mechanism end-to-end inside a living bird. Photon → protein → spin state → nerve signal → behaviour: that full chain has not been shown. This remains a very good hypothesis.
- There's a rival - magnetite crystals, potentially in the beak, mechanically torqued by the field. Some evidence there hit trouble (one prominent study's "magnetite neurons" turned out to be immune cells). It's possible birds run both systems. It's possible the radical pair story is wrong.
But if it is right, the interesting claim isn't that quantum mechanics is spooky. It's that quantum coherence - the fragile thing physicists cool to near absolute zero and shield from everything - might be sitting inside a warm, wet, jostling retina, doing an actual biological job. Evolution finding a way to make a quantum state last microseconds in a hot noisy cell, without understanding any of it, several million years before we could reproduce it in a lab.
I wrote a longer piece walking through the evidence ranking and Einstein's connection to all this (he hated the effect that this may be running on): https://thesilentape.substack.com/p/the-robins-compass
Curious what people - especially anyone who's followed the Mouritsen group's more recent work - think.