Timing of circadian shifts in light/dark preference by migratory bats supports targeting of sunset for magnetic compass calibration

Bats are nocturnal mammals whose migrations depend on the precise timing of activities to exploit favourable conditions, such as tailwinds and high insect activity, while evading predators. Understanding how migratory bats use sensory cues within their roosts to time emergence requires knowledge of the interplay between circadian rhythms, spatial cognition, and seasonal demands. We examined light-dark choice in tree-dwelling Nathusius pipistrelles (Pipistrellus nathusii) over an 18-hour cycle using a Y-maze. Bats preference for the lit exit increased prior to sunset, then switched back to preferring darkness only near sunrise. This behaviour contrasts with that of previously studied cave-dwelling bats. Irrespective of their exit choice, bats used echolocation in all trials, consistent with a multimodal sensory strategy in which light conveys critical environmental information beyond the range of echolocation, but not replacing it. However, light governed the time-dependent shift in exit selection. The circadian shifts in light/dark preference are consistent with two hypotheses related to navigation mechanisms and stopover behaviour: Preference for light cues may facilitate compass calibration mechanisms, whereas the dawn preference for darkness likely reflects roost-searching and predator avoidance. These findings underscore the integral role of vision in bat navigation and highlight how circadian rhythms modulate photic responses in a migratory context. Such insights are essential for designing wildlife-friendly lighting and for interpreting future multi-sensory experiments, including those probing bat magnetoreception, where natural photic responses must be taken into account.