  The radical pair (RP) mechanism, which describes the quantum dynamics of a
spatially separated electron pair, is considered as one of the principal models
of avian magnetoreception. Different from the conventional phenomenological
approach where the sensitivity of avian magnetoreception is characterized by
the singlet yield $\Phi_{S}$, we introduce the quantum Fisher information
(QFI), which represents the maximum information about the magnetic field's
direction extracted from the RP state, to give a precise measure of sensitivity
of avian compass essentially. The consistency between our results and
experimental observations suggests that QFI plays a decisive role in avian
magnetoreception. Besides, within the framework of quantum metrology, we can
judge the feasibility of any possible measurement scheme for avian
magnetoreception, and shed light on an intrinsic relevance between the singlet
yield and a concrete measurement scheme of our approach. The present work
allows us to understand many things about avian magnetoreception from a fully
new perspective of quantum metrology, and provide a new route to establish a
direct connection between quantum information and many other biological
functions.
