How do birds sense Earth's magnetic field?

The leading explanation is surprising: many migratory birds seem to sense Earth's magnetic field with their eyes, through a quantum effect. A protein in the eye called cryptochrome catches blue light. The light kicks an electron loose and creates a pair of molecules whose electrons are quantum entangled, a so-called radical pair. How that pair behaves depends on the magnetic field around it, and Earth's field, about 0.5 gauss, is so weak that this is currently the only credible way it could trigger chemical changes in a body.

The clues pile up. Migratory robins carry a version of cryptochrome, Cry4a, that is far more sensitive to magnetic fields than the same protein in stay-at-home pigeons and chickens, and its levels peak during spring and autumn migration. Birds also can't tell when the field is flipped 180 degrees, which an iron compass needle would notice at once.

The most striking clue came from Henrik Mouritsen's lab. His caged robins, which normally face their migration direction, couldn't orient in his wooden huts on campus. He suspected faint radio-frequency noise from nearby electrical equipment and lined the huts with aluminium, which blocks electrical noise but not magnetic fields. With the sheeting earthed, the robins oriented correctly. Without it, they turned at random. An iron compass wouldn't care about such noise. A delicate quantum compass would.

The story isn't closed: a 2025 study found evidence that pigeons sense magnetic fields in their inner ears too.