G. Blouin-demers et Pj. Weatherhead, Habitat use by black rat snakes (Elaphe obsoleta obsoleta) in fragmented forests, ECOLOGY, 82(10), 2001, pp. 2882-2896
Declining nest success of forest birds in fragmented habitat has been attri
buted to increased nest predation. Better understanding of this problem and
potential solutions to it require information on why nest predators are at
tracted to habitat edges. Toward this end we investigated habitat use by bl
ack rat snakes (Elaphe obsoleta obsoleta), an important avian-nest predator
in eastern deciduous forests. We radio-tracked 52 black rat snakes for per
iods 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 s
hedding were significantly associated with habitat edges. Gravid females lo
st an average of > 20% of their body mass, while nongravid females and male
s did not lose mass, suggesting that edges were not used because they offer
ed 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 foun
d traveling more often when located in forests. Because our results collect
ively are most consistent with the hypothesis that rat snakes use edges for
thermoregulatory reasons, the negative impact of the snakes on nesting bir
ds may be coincidental; the snakes primarily use edges for reasons other th
an foraging but opportunistically exploit prey they encounter there. Rat sn
akes appeared to respond to the edge structure rather than to how the edge
was created (natural vs. artificial). Thus, fragmentation of forests by hum
ans has created habitat structurally similar to that preferred by rat snake
s in their natural habitat, thereby inadvertently increasing contact betwee
n the snakes and nesting birds.