Diurnal activity in an insectivorous bat during migration period

Diurnal flight activity in otherwise strictly nocturnal bats has typically been linked to random disturbance from day roosts, an urgent need to balance food shortage caused by adverse weather during nighttime, or the absence of diurnal predators. However, migration may be another reason why bats fly during daylight, at least in some areas. Using community-science data collection, we obtained more than 500 records of over 15,000 bats displaying diurnal flight activity, suggesting that it is relatively common in Central Europe. The vast majority of sightings were of common noctules (Nyctalus noctula), with most records concentrated in spring and autumn. The seasonal dynamics of diurnal flights exactly coincided with migratory periods, and directional movements in autumn—when diurnal activity was most frequent and included highest numbers of observed bats—suggest that the behavior may ultimately be linked to migration ecology. The highest frequency of diurnal flights in autumn coincided with highest body mass in the studied territory, thereby refuting the hypothesis of early roost emergence due to poor body condition or decreased predation risk related to increased maneuverability. A shift from strictly nocturnal to partly diurnal activity may balance increased energetic demands imposed by migration, which is temporally synchronized with periods of cold nights when prey density is limited. Common noctule diurnal activity during the migratory period may be beneficial as they can acquire energy by foraging on daily abundant prey while saving nighttime for long endurance flights—alternatively, they may forage on the way to their migratory destination, thereby saving time. Predation risk from diurnal predators may be significantly decreased by choosing high flight altitudes, as observed particularly during autumn. We suggest that observations on the geographic distribution of diurnally flying noctules may help identify migration corridors.