Trapping grids were established in 13-ha stands, with traps placed at 40-m intervals in a 7 x 9 or 8 x 8 design. A tree-mounted or ground-placed Tomahawk 201 live trap (Tomahawk Live Trap Co., Tomahawk, Wis.) was placed at each grid point and was baited with a mixture of peanut butter, oats and molasses. Trapping occurred from late September to early November of 1994 and 1995, in conjunction with the arboreal rodent trapping sessions for the DEMO project. I conducted additional trapping sessions, as needed, during December and January of both winters to supplement the number of collared squirrels. Trapped squirrels were weighed, sexed, and ear-tagged for identification.
Twenty-seven squirrels (14 male, 13 female) were collared in 1994-95 and twenty-nine (15 male, 14 female) were collared in 1995-96. Each squirrel was equipped with a 5-g, modified SM-1, AVM radio collar (AVM Instrument Co., Livermore, CA). Only squirrels weighing greater than 100 grams were collared. Based on previous studies conducted in the Oregon Coast Range and Cascades (Rosenberg and Anthony 1992, Biswell, personal comm.), squirrels greater than 100 grams in the fall were considered adult flying squirrels. Squirrels collared in the first year of the study were not collared in the second year of the study. Collars were functional for about six months and could be detected at distances up to approximately 300 m.
After collaring, each squirrel was held overnight for acclimatization to the collar and to allow me to inspect, re-fit or remove the collar as necessary before release of the animal. Squirrels were released at the grid point of capture the following day and were allowed a minimum of one day to re-establish themselves in or around the stand before tracking began.
I located squirrels during daylight hours (during the inactive period of the squirrel), utilizing a Telonics scanner/ receiver (Telonics Telemetry-Electronics Consultants, Mesa, Ariz.) and yagi H-antenna. Tracking was conducted for the life of the collars, approximately from September to April. Each squirrel was located and its den site located approximately every 14 days (range = 1 - 22).
Species of den tree was identified and diameter at breast height (dbh), total height of the tree, height to the den (if identified) and percent lean of tree were measured with a d-tape and a clinometer. Decay stage was determined based on Cline et al. (1980), as modified by Halpern (1996). The type of den was identified, if possible, as a cavity or external nest (witches broom, epicormic branch platform, moss/lichen nest). All snags used by squirrels were assumed to have cavity dens because no other structures were ever identified on snags. Den type was listed as unknown in live trees which no cavity or external nest was identifiable. Orientation of identifiable dens was obtained with a compass. By banging or scratching on the tree, I attempted to prompt squirrels to show themselves at the entrance of dens to obtain visual confirmation of the exact den location/ site. Den tree locations were plotted by measuring distance and bearings from grid points or with Global Positioning Systems (GPS).
I chose 174 random locations (58 per site) from within the mature stands that contained den sites. I accomplished this by overlaying a grid of points (grid spacing at 20-m intervals) on a map of the study sites and used a random number generator to select random points. These random points were then located in the field and the live or dead tree > 25 cm dbh and > 3.35 m tall nearest to the random location was chosen as the plot center. Trees located at the center of random plots were identified to species and decay stage and measured for dbh, height, and percent lean.
Habitat variables surrounding each random and den tree were measured in 13-m and 25-m radius plots and were centered on the random or den tree. These two plot sizes were chosen because it was not known if, and at what scale, habitat characteristics around den trees were important to their selection by squirrels as den sites. Basal area of live conifer, dead conifer, live hardwood and dead hardwood trees were measured separately with a 20 BAF prism. All trees greater than 10 cm dbh within the 13-m radius plot were measured for dbh and identified to species and live/ dead status for use in calculation of stand density index (SDI) (Reineke 1933, Hibbs and Carlton 1989, McTague and Patton 1989). Canopy closure was mapped on graph paper within the 13-m radius plot, but was ocularly estimated within the 25-m radius plot. Moosehorn measurements (Robinson 1947, Bonnor 1967, Bunnell and Vales 1990) of canopy closure were taken at the beginning of each day (18% of all plots taken) in order to assist in "calibrating my eye" for ocular estimates. A moosehorn measurement was determined by averaging measurements obtained at 8 locations (N, NE, E, SE, S, SW, W, NW), 18 m from plot center. The shortest distance from the plot center tree to an opening was estimated. An opening was defined as any canopy opening greater than or equal to the diameter of an average dominant tree crown in the canopy. The diameter of average dominant tree crowns in each stand was ocularly estimated. Nearest canopy openings were classified as a canopy gap, road, clearcut, shelterwood, skid trail, stream or other. Direction to the nearest opening was obtained with a compass. Vertical stand structure was estimated in the 13-m radius plot by estimating the height to each distinct canopy level. The percent cover at that particular level was then estimated to the nearest 10% (Chambers 1996, McGarigal 1993).
Airstrip site - ~60 miles E. of Roseburg on Hwy 138, SW of Sec. 2, T27S, R3E. Code = AIR
Fish Cr. site - ~60 miles E. of Roseburg on Hwy 138, NE of Sec. 11, T27S, R3E. Code = FIS
Mowich site - ~71 miles E. of Roseburg on Hwy 138, SE of Sec. 32 and SW of Sec. 33, T26S, R5E. Code = MOW
