# On the functional window of the avian compass

## TL;DR

This paper investigates the radical-pair model's ability to explain the avian compass's functional window, its adaptability, and the limitations in quantitatively matching experimental data, suggesting environmental factors may be involved.

## Contribution

It demonstrates that the radical-pair model can qualitatively explain the avian compass features and explores the potential for environmental coupling to account for the functional window's width.

## Key findings

- Radical-pair model explains the functional window qualitatively.
- Hyperfine parameter adjustments can tune the functional window.
- Quantitative match of the window width remains unresolved.

## Abstract

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.