  The functional window is an experimentally observed property of the avian
compass that refers to its selectivity around the geomagnetic field strength.
We show that the radical-pair model, using biologically feasible hyperfine
parameters, can qualitatively explain the salient features of the avian compass
as observed from behavioral experiments: its functional window, as well as
disruption of the compass action by an RF field of specific frequencies.
Further, we show that adjustment of the hyperfine parameters can tune the
functional window, suggesting a possible mechanism for its observed
adaptability to field variation. While these lend strong support to the
radical-pair model, we find it impossible to explain quantitatively the
observed width of the functional window within this model, or even with simple
augmentations thereto. This suggests that a deeper generalization of this model
may be called for; we conjecture that environmental coupling may be playing a
subtle role here that has not been captured accurately. Lastly, we examine a
possible biological purpose to the functional window; assuming evolutionary
benefit from radical-pair magnetoreception, we conjecture that the functional
window is simply a corollary thereof and brings no additional advantage.
