Roost selection by insectivorous bats in temperate regions is presumably influenced by roost microclimates in relation to thermoregulatory strategies, but few studies have included temperature measurements in habitat selection models. Rocky landscape features are an important source of roosts that provide both shelter from predators and beneficial microclimates for bats. Most information about rock-roosting bats is derived from western North America. We studied microhabitat selection by Eastern Small-footed Bats (Myotis leibii) on natural talus slopes and human-made stone structures in the Appalachian Mountains of Virginia and New Hampshire, relative to thermal and structural characteristics of rock crevices. Roosts were located with a combination of radio-telemetry and randomized visual surveys. Roost switching behavior and structural characteristics of roosts did not appear to be influenced by the methods we used to locate roosts. Compared to random crevices, both sexes selected crevices with narrow openings, likely to provide protection from predators. Reproductive females also selected larger rocks and more stable microclimates; whereas males selected crevices that were structurally similar to but warmed more during the day than random crevices. Rock size and other structural characteristics influenced temperatures of roosts and random crevices alike by inhibiting excessive daytime heating and nighttime cooling. Because large rocks were associated with roost selection by reproductive females, and talus slopes with large rocks could be limited, we recommend including rock size as a variable in landscape scale habitat assessments for Eastern Small-footed Bats. Protecting or managing for habitat features with large rocks that receive high solar exposure could benefit Eastern Small-footed Bats and perhaps other rock-roosting species.

Free-ranging Eastern Small-footed Bats were located in their roost sites either using radio-telemetry or with visual searches. Roosts were categorized according to what could be inferred about the occupants when viewed from outside the crevice during visual searches, or based on physical examination in the case of captured individuals. For each roost, we measured the length, depth and height of the upper and lower rocks that formed the crevice, and the length, depth, width and orientation (categorized as horizontal, diagonal, or vertical) of the crevice. To reduce the number of variables we needed to model, we used a Principal Components Analysis to describe variation in the six rock variables, and we saved the resultant component scores as latent variables (PC 1 and PC2); these corresponded to overall size of the upper and lower rocks, respectively. Strong positive component scores indicate the largest rocks. We also recorded temperatures for a subset of crevices and for nearby ambient air on an hourly basis over 3 consecutive days to assess microclimates. Temperature variables in the dataset include 3-day averaged absolute measurements (average, maximum and minumum) and 3-day averaged relative measurements (calculated as temperature of the crevice minus ambient temperature) for overall, and at the hottest and coldest part of the day (relative average, relative minimum, and relative maximum). Temperature measurements were obtained after bats had vacated the roost and thus they portrayed the general thermal characteristics of the crevice, not the temperature when occupied. We collected the same structural and thermal measurements from a near equal number of randomly selected rock crevices within the same rock formations. 

The data files are .csv format, which are compatible with most spreadsheet software packages.