European robins find their way to North Africa each autumn by sensing the Earth’s magnetic field. The mechanism is in their eyes. A protein called cryptochrome sits in the retina; when blue light strikes it, it produces a pair of electrons whose spins are quantum-correlated. The Earth’s magnetic field tilts the balance between two possible spin states, the protein’s chemistry forks accordingly, and the bird’s nervous system reads the fork.
So the magnetic sense isn’t a separate organ. It’s tied to vision. When the bird looks toward magnetic north, something in its visual field is subtly different from looking south. The best guess is that the bird perceives a directional overlay — a fainter or brighter patch on one side — which we can only describe in words because we have no comparison.
The robin isn’t doing physics. It’s a bird. But the chemistry it’s running on needs quantum coherence to last on the order of microseconds in a warm, wet biological system. That’s longer than physicists would have guessed possible. The radical pair mechanism is one of the cleanest known cases of quantum effects mattering at biological scale.
In some experiments, when the right eye is covered the bird can’t orient by the magnetic field. The left eye doesn’t suffice. The asymmetry has been observed and isn’t explained.