  The radical pair model has been successful in explaining behavioral
characteristics of the geomagnetic compass believed to underlie the navigation
capability of certain avian species. In this study, the spin dynamics of the
radical pair model and decoherence therein are interpreted from a microscopic
state transition point of view. This helps to elucidate the interplay between
the hyperfine and Zeeman interactions that enables the avian compass, and the
distinctive effects of nuclear and environmental decoherence on it. Using a
quantum information theoretic quantifier of coherence, we find that nuclear
decoherence induces new structure in the spin dynamics without materially
affecting the compass action; environmental decoherence, on the other hand,
completely disrupts it.
