Influence of winter weather conditions on lesser short-tailed bat (Mystacina tuberculata) activity in Nothofagus forest, Fiordland

Winter activity of the New Zealand lesser short-tailed bat (Mystacina tuberculata) was monitored in temperate beech (Nothofagus) rainforest in Fiordland during winter in 1997 and 1998. Thirty-three full nights (486 h) were sampled using automatic bat detector units at 12 sites. Activity was detected on 18 nights (264 bat passes total, on 54.5% of nights). Index counts (bat passes per night, and passes per hour) were low and highly variable, implying larger sample sizes are needed before more precise estimates of activity levels can be attempted. A logistic regression model indicated that the probability of bats being active in winter increased with overnight mean temperature, and if it was raining. Dusk temperature, minimum overnight temperature, moon visibility, and significant cloud cover were excluded from the final model. Lesser short-tailed bats were still active down to -1.6 degrees C, a lower temperature than is tolerated by many other bat species reliant on flying insects. The observed winter activity followed a pattern of nights with low or no activity interspersed with nights of high activity. This is consistent with patterns observed in other lesser short-tailed bat populations, although in the Eglinton Valley, they were generally less active over the winter months. We suggest variation in winter activity levels between populations is most likely driven by climate, the quality of foraging habitat, and the physiological constraints of body size on torpor.