Selection of drey sites by Abert's squirrels in an introduced population

Abert's squirrels (Sciurus aberti) are reported to depend on ponderosa pine (Pinus ponderosa) for food and nest sites. Introduced Abert's squirrels in the Pinaleño Mountains of Arizona, however, occupy mixed-conifer forests that contain almost no ponderosa pine (about 2%). We examined selection of drey sites in this introduced population. Dreys (i.e., spherical nests) were built adjacent to the trunk at 75% of the tree height. Dreys were found in 5 different conifer species and <2% were in ponderosa pine. Drey trees were larger and had more access routes than did random trees. Drey sites were steeper, had more large trees, Douglas-fir (Pseudotsuga menziesii), and southwestern white pine (Pinus strobiformis), and less corkbark fir (Abies lasiocarpa var. arizonica) than random sites. The structural characteristics of drey trees in the Pinaleños population also were very similar to drey trees used by natural populations of Abert's squirrels in ponderosa pine forests. Our results suggest that the dependence of Abert's squirrels on ponderosa pine is not as strong as previously reported. Structural features such as tree size and access routes appear to be more important to selection of drey sites than tree species.