Klickitat County in south-central Washington supports the largest of Washington's 3 remaining populations of western gray squirrels. The region has substantial ponderosa pine and Oregon oak woodlands that represent key habitats for the species, providing both forage and nesting structure. Most of the habitat in this area is in private ownership, either as small landholdings or large industrial timberlands. The Klickitat Wildlife Area (KWA), located in the core of western gray squirrel range, supports a variety of vegetation communities including substantial pine/oak woodlands. Western gray squirrels a focal species for management of the KWA. In 1998 we began a multi-year study with the goal of obtaining baseline information on this population.
- Graduate Student Mary Linders (University of Washington) examined home range and habitat use by western gray squirrels on KWA from 1998-1999. This project was the first quantitative assessment of squirrel movements and resource use in Washington.
- In spring of 2000 we expanded the study to include a second study site on KWA and an additional site on private timberlands. Focus of the research shifted to quantifying parameters necessary to examine population growth and assessing how they may be influenced by natural and human-caused events. Parameters under investigation included: adult and juvenile survival, annual reproductive success, juvenile dispersal, adult movements and home range, and seasonal shifts in abundance. Ancillary investigations included: nest site selection, patterns of nest use, annual shifts in availability of oak and pine mast, seasonal shifts in food use, and genetic diversity.
- Western Gray Squirrels on KWA favored conifer-dominated stands over mixed oak-conifer and pure oak stands. Squirrels typically used areas with a pine overstory and an open understory. More specifically, occupied stands often were dominated by a multi-layered canopy of ponderosa pine with an upper canopy layer >14 m (46 ft) and a sparse understory of oak with little or no shrub or ground vegetation.
- Home ranges for males and females were greater in area than those reported for populations in Oregon and California. Estimates of annual home range size on KWA, based on 95% Fixed Kernel estimates, averaged 73.9 ± 16.9 SE ha (n= 9) for males and 22.1 ± 2.6 SE ha for females (n=12).
- Annual survival of adult western gray squirrels ranged from 44 to 88% for the period 1999-2005. Predation was the major cause of mortality for adult western gray squirrels in 6 of the 7 years, with isolated cause-specific mortality rates ranging from 26 to 39%.
- Notoedric mange, a disease caused by mites that burrow into the skin, was evident in the population all years and was most prevalent in the late winter and spring. Proportion of captured animals showing signs of mange averaged 19% in spring and 4% in fall. Mange reached its highest incidence in spring of 2003 (32%, n = 19).
- Most females (including one-year olds) bred each year, attempting at most 1 litter/year. Litter size averaged 3.3 and ranged from 2-5. The number of young raised to emergence age (approx. 8 weeks old) averaged 2.5/female/year and ranged from a high of 4.0 in 2000 to a low of 1.6 in 2004.
- Seasonal foods used by western gray squirrels included acorns, pine seeds, Douglas fir seeds, and hypogeous fungi (truffles). In late winter and early spring, squirrels were observed feeding on both larvae and adult rain beetles (Pleocoma spp.), a behavior previously undocumented.
- Ongoing analysis of microsatellite markers suggests that western gray squirrel populations in Washington are different from those in more southerly parts of the speciesí range. Additional samples from the populations in the North Cascades and Puget Trough will be run in 2010 and used in a comprehensive analysis of the population structure in Washington.