Short-term response of songbirds to experimental thinning of young Douglas-fir forests in the Oregon Cascades

Commercial thinning has the potential to increase structural diversity in managed conifer stands and redirect development of young stands towards structure characteristic of late-seral habitats. Thinning to increase diversity, however, is likely to require different strategies than thinning to maximize timber production. To prescribe thinning regimes that will promote diversity, managers need more information on response of wildlife to a range of thinning intensities and patterns. We studied the response of forest songbirds to three different intensities and patterns of thinning in 40-year-old stands dominated by Douglas-fir (Pseudotsuga menziesii) in the Oregon Cascades. We estimated densities of songbirds for 2 years before and 4 years after experimental thinning with standard point count methodology. We compared changes in density before and after thinning between each thinning treatment and the control with repeated measures analysis of variance. Thinning increased species richness and the density of 10 species. Furthermore, the frequency of detection of four additional species increased in thinned stands. Thinning decreased the density of five species, but no species was excluded by thinning. Our results were largely consistent with those from other studies of bird response to thinning from different regions of the Pacific Northwest. We conclude that commercial thinning rapidly promotes diversity of breeding songbirds in young, conifer-dominated stands. However, we suggest using a variety of thinning intensities and patterns, ranging from no thinning to very widely spaced residual trees, in order to maximize avian diversity at the landscape scale and structural diversity both within and among stands.