  The Radical Pair Mechanism can help to explain avian orientation and
navigation. Some evidence indicates that the intensity of external magnetic
fields plays an important role in avian navigation. In this paper, based on a
two-stage strategy, we demonstrate that birds could reasonably detect the
directions of geomagnetic fields and gradients of these fields using a
yield-based chemical compass that is sensitive enough for navigation. Also, we
find that the lifetime of entanglement in this proposed compass is
angle-dependent and long enough to allow adequate electron transfer between
molecules.
