The robin has a compass. It is in the eye, not the head.
A protein in the retina catches a photon, flips an electron, and holds two correlated spins for a few microseconds. The spins precess in whatever magnetic field is around — including Earth's. Their geometry tilts the chemistry one way or the other.
The bird does not measure a vector. It sees a brightness modulation, smeared across the visual field, brighter one way, dimmer another. Move the mouse, or tilt the phone.
The molecule is cryptochrome 4. The photon hits a flavin (FAD); an electron jumps from a nearby tryptophan; the pair sits in a singlet state and oscillates between singlet and triplet character at a rate set by the surrounding field. Singlet and triplet recombine to different products. The yield modulates with orientation.
Coherent spin oscillation in a warm wet protein. Textbooks said decoherence should kill it in nanoseconds. It lasts microseconds. Long enough to encode an angle.
Robin CRY4 is more sensitive than chicken CRY4. Selection reaches all the way down to the half-life of a spin-correlated electron pair.
This page is a metaphor, not a simulation. The real signal is far subtler than what you see here. But the shape of the experience — brightness as a function of orientation, overlaid on vision — is the right shape.