Abstract from: Feen, J. S.  1997.  Winter den sites of northern flying squirrels in Douglas-fir forests of the south-central Oregon Cascades.  M. S. thesis.  Oregon State University, Corvallis, OR.  45 pp.

I studied selection of winter den sites by northern flying squirrels (Glaucomys sabrinus) in forests of the Oregon Cascades. Fifty-six squirrels were radio-collared in three managed, 80- to 130-year-old Douglas-fir stands on the Umpqua National Forest during the winters of 1994-95 and 1995-96. Squirrels were located at their den sites approximately every two weeks from September to April. I compared characteristics of 134 winter den sites with characteristics of 174 randomly selected sites. Mean diameter at breast height (dbh) was greater for den trees than for trees at random sites (X_den = 74.0 cm, X_random = 51.6 cm, p = 0.0001) as was mean percent lean (X_den = 4.3%, X_random = 1.9%, p = 0.0001). Basal area of dead conifers around den trees was greater than around randomly selected trees (X_den = 7.9 m2/ha, X_random = 3.5 m2/ha, p = 0.0001). Canopy closure within a 13-m radius plot surrounding den trees was lower than around random trees (X_den = 57.8%, X_random = 65.1%, p = 0.0001). Squirrels used snags in much greater proportion (44%) than they were available (5%) (p < 0.0001, X² = 69.1, df = 1). I developed a logistic regression model to predict the probability of a tree containing a den. The model correctly classified 71% of the den trees and 88% of the random trees. The probability that a tree contained a winter den increased with increasing dbh and increasing lean of a tree, and if the tree was dead. Managers should consider the creation and retention of dead, larg dbh, and leaning trees as sites for winter dens for flying squirrels.