From Physics World:
Scientists believe that some species of birds navigate using Earth's magnetic field – an idea known as magnetoreception that is backed up by experiments that show that captive birds will respond to changing magnetic fields. Understanding how this happens is tricky. Although electron spins in biological molecules are affected by the Earth's magnetic field, the size of the effect is so small that it should be completely washed out by thermal fluctuations. However, some quantum systems can be extremely sensitive to external magnetic fields, and this is why scientists believe that some birds could navigate by making quantum measurements.
One such bird is the European robin, which appears to have magnetoreceptor molecules located in its visual system. Physicists believe that the measurement process is triggered when a "cryptochrome" protein absorbs light. This causes a flavin adenine (FAD) nucleotide on the protein to form an excited singlet spin state, which involves two electron spins with a combined spin of zero. This state then decays in picoseconds to a "radical pair state" in which the spin of one of the FAD electrons is transferred to an amino acid that is located about 1.5 nm away along the length of the protein.
This transfer is believed to preserve quantum coherence and because each spin is isolated from its surroundings, the resulting radical pair remains in a coherent quantum state for times greater than 10 ns. This, physicists believe, should be long enough for a robin to make a quantum measurement.
The direction in the protein along which this separation occurs provides a spatial reference for measuring the Earth's magnetic field. In particular, the relative orientation of the separation direction and the Earth's field affects the rate at which the radical pair will decay to a protonated state that provides a signal to the bird's nervous system. Scientists believe that it is this protonated state – or subsequent chemical reactions – that links the bird's sensory system to the magnetic-field measurement.