Quality of cavity microclimate as a factor influencing selection of maternity roosts by a tree-dwelling bat, Chalinolobus tuberculatus, in New Zealand.
Cavity quality is important for the productivity and survival of many species of tree-dwelling wildlife. Intensive land management practices, such as logging and agriculture, frequently reduce cavity availability and potentially affect the long-term viability of populations. The New Zealand long-tailed bat Chalinolobus tuberculatus selects roosts in small knot-hole cavities with specific structural properties relative to available cavities. They also change roosts daily among a large pool of different roosts. Such behaviour is likely to make C. tuberculatus vulnerable to human-induced deterioration in roosting habitat. This study represents a case study of the degree of sophistication sometimes required to assess availability and quality of roost sites, by testing whether roosts selected by C. tuberculatus also have specific microclimates. Selection for microclimate was demonstrated by comparing temperature and humidity inside unoccupied maternity roosts with available, apparently unused, knot-hole cavities, large trunk-hollows and ambient conditions. Compared with ambient conditions, roost and available knot-hole cavities had stable microclimates displaying only small ranges in temperature and humidity. Temperature inside cavities was lower than ambient temperature in the day and was warmer (and peaked) at night. Humidity in cavities was constantly high. Mean temperatures within trunk-hollows (not known to be used by C. tuberculatus) were cooler than mean ambient and roost temperatures, and temperature ranges in hollows were large and fluctuated similarly to ambient temperatures. Compared with available cavities and hollows, roost cavities had higher minimum temperatures, and maximum temperatures occurred significantly later in the day and continued for significantly longer. Humidity ranges were less and high humidity was maintained for longer. The results suggest that C. tuberculatus selects maternity roost sites with microclimatic conditions that are likely to accrue substantial energetic benefits. Predicted energy savings for adult bats using roost cavities compared with available knot-holes were 1.1-3.3%, and compared with hollows 3.4-7.3%. Greater energy savings would occur at night and benefit non-volant young. In order to evaluate adequately and mitigate the full impacts of land use practices, there is a need for wider tests to provide direct evidence of interactions between habitat management, cavity provision and survival of cavity-dependent wildlife.