Why Do Brown Long-eared Bats (Plecotus auritus) Fly in Winter?

We investigated the daily food and water consumption of a captive colony of three brown long-eared bats (Plecotus auritus) for 51 d. The bats were kept in a free-flight enclosure exposed to the natural photoperiod and temperature during the winter (January to March 1991) at 57° N. Water was always available, but food was available only on some nights. The mean daily temperature inside a wooden box provided as a hibernaculum was positively correlated with and slightly elevated above (0.6°-2.8° C) the mean daily temperature outside the box in the free-flight enclosure. The mean temperature inside the hibernaculum was 7.1° C and outside was 5.6° C. The mean relative humidity in the hibernaculum was 82% (range 67%-93%). The activity of the bats outside the hibernaculum was monitored by two Doppler radar units. The daily probability of an individual bat emerging from the hibernaculum was between 0.26 and 0.99. Emergence probability increased when there was food available and when it was warmer. The activity of the bats was strictly nocturnal, initial emergence occurring a mean of 64.4 min after sunset (n = 42, SD = 27.0 min). When denied access to food, the bats drank an average of 0.20 mL · bat⁻¹ · night⁻¹ on the nights that at least one emerged (n = 14 nights, SE = 0.05, range = 0.00-0.68). On warmer nights the bats were more active and ate and drank more than on colder nights. We suggest that typically in P. auritus winter flights may not be induced by the onset of starvation (and hence the need to feed) or by the onset of 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.