The Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) was listed as endangered in 1987 because its population was small and declining, its range was restricted, and its remaining habitat was threatened by human activities. We measured vegetational and other physical features around middens of the Mount Graham red squirrel (n = 215) and at random locations (n = 201) in Engelmann spruce (Picea engelmannii)-cork-bark fir (Abies lasiocarpa var. arizona) and transition-zone forests to identify characteristics that distinguish midden sites from random sites. Foliage volume, canopy closure, basal area, tree density 40 cm in diameter at breast height (dbh) were greater (P < 0.005) at midden sites than at random sites in both forest types
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Smith et al. (1994) studied this question.