Suitable over-wintering habitat is critical to the survival of snake populations at higher latitudes. The identification and protection of traditional, communal hibernation sites (hibernacula) is important for the conservation of threatened species, while the assessment of hibernacula availability may help determine the extent to which population distributions are limited by habitat suitability. In this paper, we quantified surface habitat characteristics of 10 hibernacula and the composition of basking trees used by a threatened population of black rat snakes (Elaphe o. obsoleta) at the northern limit of the species' range. Hibernacula were typically situated on relatively rocky, south-facing slopes. The co-occurrence of these features was sufficiently unique as to distinguish hibernacula from (1) a series of random sites, but not from (2) a set of intuitively identified "potential hibernacula" in the surrounding landscape. This implies that additional requisite elements (e.g., underground structure and micro-climatic conditions), which we were unable to quantify, set actual hibernacula apart from sites that appear to be otherwise suitable (i.e., "potential hibernacula"). Basking trees found at hibernacula tended to be relatively large and decayed or dying with numerous cavities. Unused trees exhibiting these characteristics were also available at both random sites and potential hibernacula indicating the suitability of those sites for basking. Our results suggest that rat snake hibernacula cannot be predictably located by simply searching for key surface habitat features in the landscape. The current data are equivocal as to whether or not this most northern population is limited by the availability of suitable over-winter habitat since the possibility that essential subterranean features of hibernacula are limiting remains to be tested. We recommend the continued use of radio-telemetry to identify and protect additional hibernacula, the preservation of basking trees at known hibernacula, and further research to determine the internal structure and micro-environments of hibernacula.
Structural habitat variables of Hibernaculas are quantified.