Design modifications affect bat box temperatures and suitability as maternity habitat.
Artificial roosting structures (e.g. bat boxes) are widely used as conservation tools for many animals, including bats. Although it is relatively easy to monitor bat box temperatures, we know little about the effect of design on temperatures within a box. Box microclimate affects energy budgets and physiological processes and, thus, suitability as a roost. Optimal temperature varies during the period when reproductive females aggregate to rear pups; warm roosts enhance pup development during gestation and lactation, while cool roosts facilitate energy savings by torpor, which is often important during post-lactation. To better understand the relation of design to internal temperature, we simultaneously compared 20 box designs (19 variations of a rocket box and one three-chamber flat box) in an open site, May to September 2018. We measured temperatures at the top, middle and bottom of each box and tallied counts of daytime and nighttime cool (≤30°C; TCOOL), permissive (30.1-39.9°C; TPERM) and stressful (≥40°C; TSTRS) temperature observations. We also measured temperature, solar radiation and wind speed at the site. We used generalized linear models with negative binomial distributions to test the effects of design, environmental variables and their interactions. Adding an external jacket or decreasing ventilation increased daytime and nighttime counts of TPERM. Increasing box volume (i.e. lengthening box by 50%) also positively affected daytime counts of TPERM, whereas decreasing box volume (by 50%) had the opposite effect. Adding an external water jacket was the only modification we tested that decreased counts of TCOOL at night. Counts of TSTRS were elevated by warmer, sunnier and less windy conditions outside, but these effects were lessened by increasing roof shading or reflectivity, adding ventilation or external jackets, or decreasing box volume. These results inform the development and implementation of novel bat box designs as conservation and management tools for maternal colonies of bats, with consideration for the effects of weather on internal temperatures.