  A quantum-based magnetic compass sensor, mediated through radical pair
reactions, has been suggested to underlie the sensory ability of migrating
birds to receive directional information from the geomagnetic field. Here we
extend the currently available models by considering the effects of slow
fluctuations in the nuclear spin environment on the directional signal. We
quantitatively evaluate the robustness of signals under fluctuations on a
timescale longer than the lifetime of a radical pair, utilizing two models of
radical pairs. Our results suggest design principles for building a
radical-pair based compass sensor that is both robust and highly directional
sensitive.
