Birds' vision may carry Earth's magnetic field through quantum chemistry in the retina
Migrating birds may literally see Earth’s magnetic field as a visual overlay produced by radical-pair reactions in retinal cryptochromes (notably CRY4). This quantum mechanism could alter reaction yields depending on field orientation, potentially guiding navigation by mapping magnetic information onto vision. While birds’ orientation changes with rotated magnetic fields and robin cryptochromes show magnetic sensitivity in vitro, direct evidence in a living bird’s receptor and brain remains unproven. A decisive test would require disabling a specific cryptochrome or electron-transfer step in a migratory bird and showing corresponding changes in magnetic orientation, while also detecting a repeatable Earth-strength signal flowing from retina to brain.
