It's been thought for a while that birds navigate using the Earth's magnetic fields. One molecule that's a candidate for their magnetic sense is called "cryptochrome." Here's my reading of their idea how this all might work: When exposed to certain colors of light, cryptochrome can become "activated," producing a pair of entangled electrons, a system in which each electron senses the other's spin direction. A magnetic field affects how long the cryptochrome stays activated. In turn, activated cryptochrome might affect how the retina sees light. This would mean the bird could see magnetic fields with its eyes.
Below is Wikipedia's entry:
According to one model, cryptochrome when exposed to blue light forms a pair of two radicals (molecules with a single unpaired electron) where the spins of the two unpaired electrons are correlated. The occurrence of such light-generated radical pairs and the correlation of the radical pair state have been confirmed recently in a cryptochrome of Xenopus laevis. The surrounding magnetic field affects the kind of this correlation (parallel or anti-parallel), and this in turn affects the length of time cryptochrome stays in its activated state. Activation of cryptochrome may affect the light-sensitivity of retinal neurons, with the overall result that the animal can "see" the magnetic field.
