Why and when do bats fly out in winter? a field study in the north of Flanders, Belgium.

We investigated winter activity of bats in two riparian areas in the north of Flanders. We recorded bat activity using automated bat detectors during 78 nights in December, January and February. Each recording we noted presence of species and presence of feeding buzzes. Feeding buzzes were only counted as a feeding buzz if we could clearly see a buzz phase II. In addition, for Pipistrellus pipistrellus we could also distinguish drinking buzzes. P. pipistrellus was observed during 63% of the nights. Myotis daubentonii was found 22-30% of the nights depending on the site. Total activity of M. daubentonii per night was very strongly correlated with feeding. For P. pipistrellus activity correlated better with drinking. This is in line with the hypothesis that species that hibernate in buildings where no water is present must fly out to drink, while species that hibernate in objects where drinking water is available fly out to feed. We modelled the total activity per night in function of weather conditions and moon phase. We used one of two temperature measurements: the temperature at sunset and the maximum temperature of the previous day. For P. pipistrellus the latter had a lower AIC. For M. daubentonii the best fit was obtained with the temperature at sunset. This is consistent with the hypothesis that P. pipistrellus can estimate the temperature through the warming of its hibernacula while M. daubentonii that hibernates in more buffered systems has to fly out. The activity of P. pipistrellus was also adversely affected by the illumination of the moon. The fact that bat activity was observed during so many nights indicates that winter activity is an essential part of bat ecology, and riparian areas are an important habitat for bats in winter. Therefore, winter activity of bats should be taken into account in management plans and environmental impact assessments, as it has its own specific conservation implications.