  We review the spin radical pair mechanism which is a promising explanation of
avian navigation. This mechanism is based on the dependence of product yields
on (1) the hyperfine interaction involving electron spins and neighboring
nuclear spins and (2) the intensity and orientation of the geomagnetic field.
One surprising result is that even at ambient conditions quantum entanglement
of electron spins can play an important role in avian magnetoreception. This
review describes the general scheme of chemical reactions involving radical
pairs generated from singlet and triplet precursors; the spin dynamics of the
radical pairs; and the magnetic field dependence of product yields caused by
the radical pair mechanism. The main part of the review includes a description
of the chemical compass in birds. We review: the general properties of the
avian compass; the basic scheme of the radical pair mechanism; the reaction
kinetics in cryptochrome; quantum coherence and entanglement in the avian
compass; and the effects of noise. We believe that the "quantum avian compass"
can play an important role in avian navigation and can also provide the
foundation for a new generation of sensitive and selective magnetic-sensing
nano-devices.
