jamie found a post on the Not Exactly Rocket Science blog on research indicating that some birds can literally see magnetic fields, but only if the vision in their right eye is sharp (abstract at Current Biology). "The magnetic sense of birds was first discovered in robins in 1968, and its details have been teased out ever since. 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. ... 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."