Some birds can probably literally see Earth's magnetic field

Migratory birds like robins can cover thousands of kilometers with astonishing navigational precision, even in overcast weather with no landmarks to go by — something that has long fascinated researchers of animal behavior.

One leading hypothesis, known as radical-pair magnetoreception, holds that a bird's retina contains special light-sensitive proteins in which Earth's magnetic field affects the chemical reactions triggered by light — effectively "overlaying" extra information about the direction of the magnetic field onto the visual image.

In other words, a bird might literally see something like a shadow or a pattern indicating the direction of the magnetic field, superimposed on its normal view of its surroundings — though nobody knows exactly what that image "looks like" from the bird's own perspective.

The mechanism relies on quantum effects occurring within single protein molecules, making it one of the few situations where quantum phenomena could have direct biological significance — but the precise mechanism has still never been conclusively confirmed experimentally.