For more information on species & ecosystem science:

Wildlife Science
360-902-2515
wildthing@dfw.wa.gov

Fish Science
360-902-2700
fishpgm@dfw.wa.gov

Habitat Science
360-902-2534
habitatprogram@dfw.wa.gov

 
 

Lead Scientist: Matt Vander Haegen

Ecoregions: Okanogan

Ecological Systems: Columbia Basin Foothill Riparian Woodland and Shrubland, Northern Rocky Mountain Ponderosa Pine Woodland and Savanna, Northern Rocky Mountain Dry-Mesic Montane Mixed Conifer Forest

   
 
 

Western gray squirrels in the North Cascades often use Ponderosa pine stands such as this one in Black Canyon Creek, Okanogan County

   
 
 

Western gray squirrel nest in a mistletoe broom, Okanogan County

   
 
 

Percentage of nest trees (black bars) vs. random trees (orange bars) in groups defined by the degree of mistletoe infection. Most random trees had no brooms, whereas trees with nests often had 1 or more brooms.

   
 
 

Relative home range areas used by female western gray squirrels in the breeding vs. non-breeding season in the North Cascades. Each number represents a different female.

   

Western Gray Squirrel Ecology

Home Range and Nest Use by Western Gray Squirrels
in the North Cascades

Project Description

Throughout most of its range, the western gray squirrel inhabits conifer-hardwood forests. In most parts of its range, the western gray squirrel is described as an oak-obligate species that depends on a diverse mixture of oak and conifer trees. When oak is present, western gray squirrels harvest acorns for food and use oak cavities for natal dens. The North Cascades of Washington represent not only the northern extent of the western gray squirrel's range, but one of the only areas where it exists without oak trees.

Fire suppression over the past 100 years in the interior dry forests of western North America has resulted in dense stands of young, suppressed trees, and increased coarse woody debris that increase vulnerability to disease and catastrophic wildfire. Increasingly, forests in this region are being managed to reduce these threats, altering the structure of stands for a variety of wildlife including western gray squirrels. Knowledge of how squirrels use the forest and what tree and stand characteristics are selected for key activities such as nesting will be critical for development of suitable management strategies.

Graduate student Sara Gregory (University of Washington) examined movements and nest use by western gray squirrels in Black Canyon Creek drainage of the North Cascades from 2003-2004. Objectives of this research were to (1) describe the characteristics of western gray squirrel nests in an area lacking oak; (2) quantify western gray squirrel nest selection relative to availability at 2 spatial scales, the nest site and nest tree; and (3) develop specific management guidelines for maintaining appropriate nesting opportunities in this unique dry forest habitat.

Key Findings

  • Without oak cavities, females reared their young in dreys. General nest-tree characteristics were similar to characteristics of western gray squirrel nest trees in south-central Washington: relatively tall ponderosa pines >40 cm diameter at breast height.
  • Results from conditional logistic models determined that odds of a squirrel selecting a tree for nesting increased with greater diameter at breast height and with infection by dwarf mistletoe (Arceuthobium spp.). All nests built in trees with mistletoe used the broom as part of the nest structure; in many cases, the broom supporting the nest was the only visible broom in the tree.
  • Nest sites with high selection probability by squirrels had greater basal area and number of tree species than available, unselected sites. Retention of patches that include a mix of conifer species or conifer and deciduous trees and moderate to high basal area could promote nesting opportunities, connectivity for arboreal travel, as well as abundance and diversity of hypogeous fungi
  • To enhance nesting opportunities for western gray squirrels, we recommend retaining trees >20 cm diameter at breast height (min. size used for nesting) with mistletoe brooms in the upper one half of the crown.
  • Estimates of annual home range size, based on 95% Fixed Kernel estimates, averaged 281.0 ± 25.6 SE SE ha (n= 4) for males and 75.2 ± 11.1 SE ha SE ha for females (n=8). Average home range size for both males and females were greater than those measured in south-central Washington.

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