Implications of Thinning for Songbirds

Joan Hagar, Forest Science, Oregon State University, Corvallis, OR

Composition of forest songbird assemblages is related to forest structural features such as vertical diversity of vegetation, tree size, and snag density. Low songbird diversity in young conifer stands that have developed following clear-cut harvesting is related to low structural diversity in these stands. Thinning has the potential to increase structural diversity in young stands, and to accelerate development of habitat for birds associated with late seral conditions. Several studies in Oregon and Washington have provided consistent evidence that bird species diversity increases following thinning of dense young conifer stands. Several species rarely observed in unthinned stands are observed consistently or are even common in thinned stands. Although some bird species respond negatively, thinning has not eliminated any species. Most species that exhibit a short-term negative response to thinning are likely to benefit from the long-term effects of thinning on stand structure. Modification of traditional evenly spaced, low intensity thinning is probably necessary to achieve a range of residual tree densities that promotes songbird diversity on stand and landscape scales. Continued research and monitoring of songbirds in thinned and unthinned stands is required to assess the adequacy of thinning as a tool for promoting development of habitat to support species associated with late seral structural attributes.