Cryptochrome proteins in bird retinas absorb blue light, creating a radical pair — two molecules each with an unpaired electron. Quantum coherence between singlet |S⟩ and triplet |T⟩ states evolves under hyperfine coupling and the external magnetic field. The singlet/triplet ratio determines the signaling state, encoding field direction.
Birds sense the inclination angle of Earth's field (dipping angle) — not polarity. This means they distinguish "poleward" (field lines dip toward pole) from "equatorward" but cannot be fooled by reversing the horizontal component alone. Disruption by radiofrequency fields at Larmor frequency (1.4 MHz for 50 μT) abolishes orientation, confirming the quantum mechanism (Ritz 2004).
The radical pair mechanism operates at body temperature — remarkable given that quantum coherence is normally destroyed by thermal noise. Decoherence time must exceed ~1 μs for sufficient field sensitivity (Schulten's threshold). Cryptochrome Cry4 shows highest expression in birds during migration season. The retinal basis means birds may literally "see" magnetic field lines.