I have previously written about birds and their uncanny ability to know what direction north is here. Both that and this post are both on the topic of birds’ ability to see magnetic fields probably caused by a magnetically sensitive cell in the eye. But the exact nature of this magnetoreception is still unknown. The two main theories are that either an iron containing particle could interact with the magnetic field to aid navigation or perhaps that light induced radical pairs of electrons could have their spins slightly biased based on an ever present magnetic field. This second idea is the one that is examined more closely as if the theory is true the mechanism would be quantum mechanical in many ways.
One of the ideas is that the magnetoreception is based on the quantum coherence between the separated pair of electrons. If this theory is true then a chemical sensor for this effect has to be discovered to convert the quantum physical action into a biological process that would grant a special sight to birds. It has been shown that cryptochrome, a branch of photosensitive proteins, are required for fruit flies to detect magnetic fields although there is no conclusion as to what this means.
In this paper the radical pairs were examined with the addition of a paramagnetic molecule. In a way, a kind of combination of the two theories. The presence of this molecule, called a scavenger, results in an increased interaction of the electron pair with a trace amount of magnetic field (it is the job of the scavenger to find traces). This means that a relatively strong field can be found even when there was none before, and even when the radical pair was separated by over 2 nanometres the ability to be used as a magnetic compass remained.