Cave visitation by temperate zone bats: effects of climatic factors

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We investigated the effects of climatic variables on the flight activity of bats at the entrance of a hibernaculum (Kateřinská cave, Moravian Karst, CZ). Activity was recorded automatically using a double infrared-light logging system. Climatic factors influenced not only seasonal but also night-to-night and overnight patterns in cave visitation. The effect of individual variables and their contribution to variability in activity levels changed during the year. (1) Flight activity during late hibernation (5 March–14 April) was positively affected by the mean ambient temperature (Tavg) and negatively affected by previous day minimal temperature. (2) During the departure period (15 April–4 June), nightly activity correlated with Tavg and Pavg (mean barometric pressure). Previous day rainfall caused a decline in the activity levels. (3) Summer activity (5 June–26 July) increased as the range of daily temperature (Tdif max−min) increased and was suppressed by previous day rainfall. In contrast, a higher amount of rainfall (>10 mm) in the study day caused an increase in activity, likely due to bats sheltering. (4) During swarming (5 September–14 November), activity was positively related to Tavg, Pavg and the amount of rainfall. (5) During hibernation (15 November–4 March), temperature (Tavg and Tdif max−min) was the best predictor of the activity level. The percentage of nights on which activity occurred increased with increasing temperature during hibernation and late hibernation. Activity occurred even at temperatures<0 °C (Tmin=−13.2 °C). The recordings were all positive at Tmax≥6.2 °C. The activity within corresponding temperature groups was significantly lower during hibernation than during late hibernation. We review possible explanations for the patterns observed.