Why do brown long-eared bats (Plecotus auritus) fly in winter?

Results suggest that winter flights may not be induced by onset of starvation (and hence the need to feed) or by dehydration (and hence the need to drink). Rather, at typical winter temperatures P. auritus may fly frequently, almost daily, to try and ensure that neither energy nor water reserves approach critically low levels. Only during a prolonged cold period (mean night temperature <4°C) might many days pass without a winter flight. -from Authors