Some birds have a heads-up display compass MAR 15 2012
Birds can detect the magnetic field of the Earth, which gives them an incredible sense of direction. Curiously, this sense of direction doesn't work in darkness. This led scientists to discover that some birds can actually see the directions overlaid on their normal vision, like a heads-up display.
According to the new model, when a photon of light from the Sun is absorbed by a special molecule in the bird's eye, it can cause an electron to be kicked from its normal state into an alternative location a few nanometres away. Until the electron eventually relaxes back, it creates an 'electric dipole field' which can augment the bird's vision - for example altering colours or brightness.
Crucially, the alignment of the molecule compared to the Earth's magnetic field controls the time it takes for the electron to relax back, and so controls the strength of the effect on the bird's vision.
There are many such molecules spread throughout the eye, with different orientations. So from the patterns on top of its vision, and the change of these patterns as it moves its head, the bird learns about the direction of Earth's magnetic field.