"Rock squirrels, with their long, bushy tails, look very much like tree squirrels, but seldom climb trees. They are most commonly found in rocky habitats—canyons, cliffs, and hillsides. Occasionally a nest is found in a tree, but they usually dig burrows, choosing a place that offers a near-by lookout where they can watch for danger. Other mammals and even burrowing owls are known to use their dens if the squirrels abandon them. The rock squirrel's geographic range is large, and it is found at elevations from sea level to 2,900 m. Females produce one litter a year in places or at elevations where winter weather lasts longer, and two in warmer parts of their range."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account