Bat Boxes as Mitigation Tools: Factors Impacting Microclimate and Myotis sodalis Roost Selection

Environmental degradation has led to declines in available natural roosting habitat for bats. To mitigate this loss, practitioners often deploy artificial roosts (e.g., bat boxes). There are no established species-specific practices for deployment strategy and roost design selection, but occupancy rates are known to vary across species and roost microclimates can be harmful to bats. Providing bats with thermally beneficial roosts during summer could enhance overwinter survival of WNS-affected species. To further our understanding of roost preference and microclimate, we deployed 40 rocket box roosts of 5 designs at field sites in Indiana and Kentucky. Roosts were deployed in clusters of 5 at 4 distinct solar exposures within each site. From April-September of 2019, we collected hourly roost microclimate data via Thermochron iButtons (12 sensors/box) and monitored occupancy of resident Indiana bats (Myotis sodalis) via spotlight checks and emergence counts 2–4 times per week. Following an information theoretic modeling approach, we used hurdle models to assess the effects of design, solar treatment, and weather on occupancy and abundance. We used linear models and beta regression to test the effects of design, solar treatment, weather, and bat abundance on roost microclimate. Indiana bats showed no preference for roost design, but preferred roosting in easterly and westerly sun roost clusters, which provide solar exposure and access to cover upon emergence. Bats were more likely to be present and more abundant under warm, calm weather conditions. Vent removal and reference designs logged the most unsuitably hot recordings across solar treatments, while unsuitably cold recordings were similar across designs and solar treatments. At low ambient temperatures (\u3c 20ºC), large groups of bats (≥ 30) had a substantial positive impact on within-roost temperature availability (hourly TMAX–TMIN) and variability (daily TMAX–TMIN) as compared to unoccupied roosts. Group size had varying effect strengths based on interactions with roost design. Further, during the summer months (June-August), 3 designs (external water jacket, chimney, and white tile roof) had microclimates more suitable for bats as compared to a reference design. Though during the cool spring months, landscape position and design had little effect on roost suitability, though further investigation is warranted. To promote warm roosting conditions and access to cover upon emergence, we recommend deploying boxes on solar-exposed tree-lined edges. Researchers should further consider the potential impact that bats may have on a prospective artificial roost before deployment, as social thermoregulation could alter box microclimates, thereby affecting bats’ energetic budgets. Additional roost monitoring is warranted as bats may or may not develop a preference for roost designs in subsequent years. Further development and testing of roost designs that can buffer against unsuitably hot and cold temperatures is likely critical to improve conservation outcomes for bats