Seasonal Movements and Nest Site Selection of the Western Gray Squirrel (Sciurus griseus) in the Methow River Watershed Sara Caroline Gregory Master of Science College of Forest Resources University of Washington December 2005 Sue Misao/Methow Valley news 2004 Seasonal Movements and Nest Site Selection of the Western Gray Squirrel (Sciurus griseus) in the Methow River Watershed Sara Caroline Gregory A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science University of Washington 2005 Program Authorized to Offer Degree: College of Forest Resources In presenting this thesis in partial fulfillment of the requirements for a master’s degree at the University of Washington, I agree that the library shall make its copies freely available for inspection. I further agree that extensive copying of this thesis is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Any other reproduction for any purposes or by any means shall not be allowed without my written permission. Signature________________________________________ Date________________________________________ University of Washington Abstract Seasonal movements and nest site selection of the western gray squirrel (Sciurus griseus) in the Methow River watershed Sara Caroline Gregory Chair of the Supervisory Committee: Professor Stephen D. West College of Forest Resources Listed as a state threatened species, the distribution of the western gray squirrel (Sciurus griseus) in Washington is limited to three disjunct areas. Little is known about the North Cascades population, which is the northernmost population for the species. Here, squirrels exist without oaks that provide winter forage and cavities for maternal nests elsewhere in its range. During May 2003 to August 2005, we studied movements and nesting habits, for eight female and four male radio- collared squirrels in Okanogan County, Washington. All movements and total home- range estimates were significantly larger than estimates reported for populations in California, Oregon, and south-central Washington. Average 95% fixed kernel home ranges were 142 ha for males and 49 ha for females, a significant difference. Fifty percent core home-range estimates for females averaged 6 ha and were not significantly different from those in south-central Washington. These results indicate that squirrels in this region are traveling increased distances to obtain necessary resources. In the absence of oak cavities, females reared their young in stick nests. General nest-tree characteristics were similar to characteristics of nest trees in Klickitat County: dominant or co-dominant ponderosa pines equal or greater than 40 cm diameter at breast height (dbh) with an interior or marginal stand position. The odds of a squirrel selecting a tree for nesting increased with mistletoe infection, dbh, and connectivity of the tree canopy. Nest sites with high selection probability by squirrels had greater basal area, larger mean dbh, and higher richness of tree species than control sites. To enhance squirrel habitat in this region, we recommend retaining trees larger than 22 cm dbh with mistletoe deformations in the upper portion of their crown that are large enough to accommodate a nesting squirrel (at least 20 cm diameter). Potential nest sites in these predominantly ponderosa pine forests should be managed for high basal area (>15 m2/ha), large mean dbh (>25 cm), and a mix of conifer and deciduous tree species. TABLE OF CONTENTS Page List of Figures.........................................................................................................iii List of Tables..........................................................................................................iv Introduction .............................................................................................................1 Objectives as Hypotheses........................................................................................9 Methods .................................................................................................................11 Study site selection and description ................................................................11 Trapping ..........................................................................................................14 Handling ..........................................................................................................15 Radio telemetry ...............................................................................................16 Nest-tree and nest-site sampling......................................................................17 Data analysis....................................................................................................22 Movements ................................................................................................22 Nesting habits ............................................................................................24 Results ...................................................................................................................32 Trapping and telemetry....................................................................................32 Interfix distance...............................................................................................32 Home-range estimates .....................................................................................34 Overlap ............................................................................................................36 Nest use ...........................................................................................................41 Nest and nest-tree characteristics ....................................................................43 Nest-tree selection ...........................................................................................47 Nest-site selection............................................................................................49 Natal nests .......................................................................................................57 Discussion..............................................................................................................62 Movements ......................................................................................................62 i Page Nest use ...........................................................................................................68 Nest and nest-tree characteristics ....................................................................70 Nest-tree selection ...........................................................................................71 Nest-site selection............................................................................................73 Natal nests .......................................................................................................75 Conclusions ...........................................................................................................77 Movements ......................................................................................................77 Nest use ...........................................................................................................78 Nest and nest-tree characteristics ....................................................................79 Nest-tree and nest-site selection ......................................................................79 Natal nests .......................................................................................................80 Recommendations .................................................................................................82 Bibliography..........................................................................................................86 ii LIST OF FIGURES Figure Number Page 1. Distribution of the western gray squirrel...............................................2 2. Washington range of the western gray squirrel with focus on the Methow River Watershed......................................................................4 3. Aerial and ground photos of the study area.........................................13 4. Orientation of sites sampled for selection analysis .............................18 5. Diagram of vegetation sampling scheme ............................................20 6. Average monthly distances traveled by male and female squirrels ....35 7. Orientation of male and female seasonal home ranges .......................39 8. Proportion of squirrel nest use by season............................................44 9. Categorical distribution of nest trees in Okanogan and Klickitat Counties...............................................................................................46 10. Distribution of control and nest trees by TBV rating..........................52 11. Probability plots for the model Stand..................................................60 12. Distribution of non-natal and natal nest trees by TBV rating .............61 13. Female core areas and natal nests........................................................67 iii LIST OF TABLES Table Number Page 1. Variables for nest-tree selection ..........................................................26 2. Variables for nest-site selection...........................................................27 3. A priori candidate models for nest-tree selection................................28