Ecology of the Western Gray Squirrel in South-Central Washington: Progress Report

We studied the ecology of the western gray squirrel (Sciurus griseus) in south-central Washington from 2000-2004, focusing on the parameters necessary to examine population growth and how they may be influenced by natural and human-caused events. We captured 149 individual squirrels during 17 semi-annual trapping session (651 total captures) and marked most with ear tags and radio-collars. Annual survival of adults ranged from 51 to 65% and was similar for males and for females. Mortality for males tended to occur more frequently during late winter and early spring, coinciding with the mating season. Survival of juvenile squirrels from first capture in fall through 1 March ranged from 60 to 86%. Predation was the major cause of mortality for western gray squirrels most years. Notoedric mange was present in the population all years and was most prevalent, and most severe, in winter and early spring. Mortality due to mange ranged from 10 to 40% annually and was the dominant cause of death one year. Most females attempted breeding each year, including yearlings. Litter size averaged 3.3 and ranged from 2-5 (n = 19). The number of young raised to emergence from the natal den (approx. 8 weeks of age) averaged 2.5 (n = 45). Density of squirrels on 3 study sites, as determined from mark-recapture estimates, ranged from 0.1 to 0.26/ha in spring and from 0.17 to 0.43/ha in fall. Home range (95% fixed-kernel) estimates for female western gray squirrels averaged 18.7 ha (9.9 sd, n = 49); 50% core use areas averaged 4.29 ha (2.25 sd, n = 49). Examination of core use area plots revealed little overlap among females within the same year, suggesting exclusive use of these core areas. Females demonstrated strong fidelity to core use areas from year to year although the size of these areas varied among years. Twenty percent of juvenile squirrels dispersed from the study area where they were captured. Dispersal rates for males (3 of 13) and for females (3 of 17) were similar. Mean dispersal distance was 2862 m (213 sd). Mast surveys of Oregon white oak and ponderosa pine revealed wide variation in availability of these foods from year to year, but similar trends between the 2 study sites surveyed. Plans for 2005 include continued demographic monitoring with an emphasis on survival and movements of young after leaving the natal den. Potential timber harvests on 2 of the 3 study sites also may provide an opportunity to examine how females respond to changes in the stand structure of their core areas and home range.