Cavity nests are rarely used by Abert's squirrels (Sciurus aberti) in ponderosa pine (Pinus ponderosa) forests. Introduced Abert's squirrels in the Pinaleño Mountains of Arizona, however, exhibit occasional use of cavity nests (ca. 10%). We examined characteristics of cavity nest sites in this introduced population. Cavity nests were located at two-thirds of the tree height and were found mainly in both dead and live aspen (Populus tremuloides). Aspen cavity trees were larger than aspen random

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... rees. Aspen cavity sites had more large snags and less ponderosa pine and were less steep than aspen random sites. Abert's squirrels possibly use cavities in large aspen trees because thick trunks and older trees might allow larger cavities to form, might provide greater thermal and wind protection, or both. The greater use of cavity nests by the Abert's squirrels in our study could be due to the higher frequency of cavity-forming aspens or harsher winter conditions in mixed-conifer forests of the Pinaleño Mountains than in ponderosa pine forests where Abert's squirrels typically occur or both.