Effects of Commercial Thinning on Breeding Bird Populations in Western Hemlock Forests

Bird populations and habitat structure were compared between three commercially thinned and three unthinned western hemlock (Tsuga heterophylla) stands to assess short-term effects of commercial thinning on breeding bird communities. Thinning reduced the density of small trees and snags (≤30 cm dbh), but did not affect the density of large trees or snags (>30 cm dbh). The overstory canopy was more open and cover of forbs, grasses and seedlings was higher in thinned than unthinned stands. Winter wrens (Troglodytes troglodytes), dark-eyed juncos (Junco hyemalis), chestnut-backed chickadees (Parus rufescens) and red-breasted nuthatches (Sitta canadensis) were more abundant in thinned than unthinned stands, but total bird density did not differ between thinned and unthinned stands. Commercial thinning thus enhances habitat conditions for some bird species while having minimal effects on other bird species.