Differential selection of roosts by Eastern Small-footed Myotis relative to rock structure and microclimate

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.

Methods
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.

温带地区食虫蝙蝠的栖息位选择，推测受与体温调节策略相关的栖息位微气候（microclimate）影响，但现有研究中极少有将温度测量纳入生境选择模型（habitat selection models）的案例。岩生景观要素（rocky landscape features）是重要的蝙蝠栖息位来源，可为蝙蝠提供躲避天敌的庇护所与适宜的微气候环境。目前关于岩栖蝙蝠的研究资料大多源自北美西部。我们针对弗吉尼亚州与新罕布什尔州阿巴拉契亚山脉（Appalachian Mountains）区域的天然岩屑坡（talus slopes）及人工石质结构（human-made stone structures），结合岩石缝隙的热学与结构特征，研究了东部小足蝠（Eastern Small-footed Bat, Myotis leibii）的微生境选择行为。研究人员通过无线电遥测（radio-telemetry）与随机目视调查（randomized visual surveys）相结合的方式定位栖息位。换栖位行为（roost switching behavior）与栖息位的结构特征似乎并未受本研究采用的定位方法影响。相较于随机选取的缝隙，雌雄个体均偏好开口较窄的缝隙——这类缝隙可有效帮助蝙蝠躲避天敌。繁殖期雌性（reproductive females）还会选择更大的岩石与微气候更稳定的栖息位；而雄性偏好结构与随机缝隙相似，但日间升温幅度更高的缝隙。岩石尺寸与其他结构特征可通过抑制日间过度升温与夜间过度降温，同时影响栖息位与随机缝隙的温度。鉴于繁殖期雌性偏好选择大型岩石作为栖息位，且含大型岩石的岩屑坡资源可能有限，我们建议在东部小足蝠的景观尺度生境评估（landscape scale habitat assessments）中，将岩石尺寸作为一项评估变量。保护或优化高日照暴露（solar exposure）的大型岩石生境特征，可惠及东部小足蝠，或其他岩栖蝙蝠物种。

研究方法
自由活动的东部小足蝠的栖息位可通过无线电遥测（radio-telemetry）或目视搜寻（visual searches）的方式定位。目视搜寻时，研究人员可通过缝隙外部观察推断栖息个体的情况；若为捕获个体，则通过身体检查确定栖息位的分类依据。针对每个栖息位，研究人员测量了构成缝隙的上下两块岩石的长度、深度与高度，以及缝隙本身的长度、深度、宽度与朝向（分为水平、斜向、垂直三类）。为减少建模所需的变量数量，我们采用主成分分析（Principal Components Analysis, PCA）对6项岩石特征变量的变异进行降维，并将得到的主成分得分作为潜变量（latent variables）（PC1与PC2）；二者分别对应上下两块岩石的整体尺寸，主成分得分越高则代表岩石尺寸越大。为评估微气候，我们连续3天每小时记录了部分缝隙及周边环境空气的温度。本数据集的温度变量包括：整体时段及当日最热、最冷时段的3日平均绝对温度（平均值、最大值与最小值），以及3日平均相对温度（以缝隙温度减去环境温度计算得到的相对值，含相对平均值、相对最小值与相对最大值）。所有温度测量均在蝙蝠离开栖息位后进行，因此记录的是缝隙的常规热学特征，而非蝙蝠栖息时的实际温度。我们在同一岩组（rock formations）内随机选取了数量相近的岩石缝隙，并采集了相同的结构与热学特征测量数据。