Robins can literally see magnetic fields — but only if their right eye can see clearly:
Years of careful research have told us that the ability depends on light and particularly on the right eye and the left half of the brain. The details still aren’t quite clear but, for now, the most likely explanation involves a molecule called cryptochrome. Cryptochrome is found in the light-sensitive cells of a bird’s retina and scientists think that it affects just how sensitive those cells are.
When cryptochrome is struck by blue light, it shifts into an active state where it has an unpaired electron — these particles normally waltz in pairs but here, they dance solo. The same thing happens in a companion molecule called FAD. Together, cryptochrome and FAD, both with unpaired electrons, are known as a “radical pair”. Magnetic fields act upon the unpaired electrons and govern how long it takes for the radical pair to revert back to their normal, inactive state. And because cryptochrome affects the sensitivity of a bird’s retina, so do magnetic fields.
The upshot is that magnetic fields put up a filter of light or dark patches over what a bird normally sees. These patches change as the bird turns and tilts its head, providing it with a visual compass made out of contrasting shades.
To test the bounds of this ability, Stapput wanted to see what would happen if she blurred a robin’s vision. She outfitted her robins with somewhat unflattering goggles, with clear foil on one side and frosted foil on the other. Both allowed 70% of light to get through, but the frosted foil disrupted the clarity of the image.
The robins were kept in cages until they were ready to migrate and let loose in funnel-shaped cages lined with correction fluid. As they orientated themselves and changed course, they created scratches on the cage walls which told Stapput which direction they were heading in. These scratches revealed that with both eyes open, the robins flew straight north as they would normally do in the wild. If their left field of vision was frosted, they went the same way. But if their right eye was covered, they became disorientated, heading in completely random directions.