Habitat use by black rat snakes (Elaphe obsoleta obsoleta) in fragmented forests

Declining nest success of forest birds in fragmented habitat has been attributed to increased nest predation. Better understanding of this problem and potential solutions to it require information on why nest predators are attracted to habitat edges. Toward this end we investigated habitat use by black rat snakes (Elaphe obsoleta obsoleta), an important avian-nest predator in eastern deciduous forests. We radio-tracked 52 black rat snakes for periods of 3-41 mo from 1996 to 1999. All black rat snakes exhibited a strong preference for edge habitats. Consistent with edges being used because they facilitate thermoregulation, gravid females associated more strongly with edges than did males and nongravid females, and sites used by snakes when shedding were significantly associated with habitat edges. Gravid females lost an average- of >20% of their body mass, while nongravid females and males did not lose mass, suggesting that edges were not used because they offered high success in foraging. Similarly, an increase in use of edge habitat through the season by all rat snakes was inconsistent with the snakes being attracted principally to hunt: avian prey would have been more abundant in spring when birds were breeding, and the density of small mammals in edges did not vary seasonally. Also, snakes moved longer distances and were found traveling more often when located in forests. Because our results collectively are most consistent with the hypothesis that rat snakes use edges for thermoregulatory reasons, the negative impact of the snakes on nesting birds may be coincidental; the snakes primarily use edges for reasons other than foraging but opportunistically exploit prey they encounter there. Rat snakes appeared to respond to the edge structure rather than to how the edge was created (natural vs. artificial). Thus, fragmentation of forests by humans has created habitat structurally similar to that preferred by rat snakes in their natural habitat, thereby inadvertently increasing contact between the snakes and nesting birds.