Response of birds to thinning young Douglas-fir forests

Abstract

Silvicultural practices, such as thinning, are increasingly performed both for commodity production and to help achieve biodiversity goals and promote ecological restoration. However, relatively little research has examined effects of thinning conifer forests on vertebrates. We experimentally manipulated stands using a randomized block design to evaluate influences of two thinning intensities on populations of diurnal breeding birds in western Oregon. We conducted point counts of birds seven times each year in 1994 (prior to treatment) and from 1995 through 2000 (subsequent to treatment). We analyzed data using multiple linear regression and information-theoretic approaches to model selection. Of the 22 species for which we had sufficient data for analysis, detections of nine species decreased and eight species increased in thinned stands relative to controls, and there was no strong evidence that thinning influenced numbers of five species. Of the 17 species that responded to thinning, the magnitude of response of eight species varied with thinning intensity; for each of these species, response was greatest in the more heavily thinned stands. Although no species was extirpated from stands following thinning, detections of Hutton's Vireos (Vireo huttoni), Golden-crowned Kinglets (Regulus satrapa), Brown Creepers (Certhia americana), Black-throated Gray Warblers (Dendroica nigrescens), and Varied Thrushes (Ixoreus naevius) decreased to less than half of the detections in controls in one or more treatment types, suggesting thinning significantly reduces their numbers. In contrast, American Robins (Turdus migratorius), Townsend's Solitaires (Myadestes townsendi), and Hammond's Flycatchers (Empidonax hammondii) were rare or absent in controls but regularly present in thinned stands, and detections of Western Tanagers (Piranga ludoviciana), Evening Grosbeaks (Coccothraustes vespertinus), and Hairy Woodpeckers (Picoides villosus) increased by threefold or more in thinned stands relative to controls. Only Pacificslope Flycatchers (Empidonax difficilis), Warbling Vireos (Vireo gilvus), and Western Tanagers showed strong evidence of temporal trends in response. For these species, differences between numbers in controls and treated stands became more extreme through time. Our findings suggest that thinning densely stocked conifer stands in landscapes dominated by younger stands enhances habitat suitability for several species of birds, but that some unthinned patches and stands should be retained to provide refugia for species that are impacted by thinning.

Stand management: a threat or opportunity for birds in British woodland?

“…Others would be the impact of different thinning regimes on the rate of transition and on the provision of the key habitat features. Impact of new management methods (such as Continuous Cover Forestry (CCF) and Low Impact Silvicultural Systems (LISS)) on stand structure and habitat features, bird populations and biodiversity. This could include direct studies of impacts of silvicultural systems on bird populations – for example, there has been considerable North American research on thinning and group selection (Annand & Thompson 1997, Easton & Martin 1998, Robinson & Robinson 1999, Moorman & Guynn 2001, Hayes et al . 2003) but no comparable work in Britain. Responsiveness of bird species.…”

Forest Restoration Using Variable Density Thinning: Lessons from Douglas-Fir Stands in Western Oregon

“…Moreover, large treatment areas allowed investigations of phenomena that act at broader spatial scales [20]. For example, treatment areas that cover multiple home ranges of songbirds enable investigations about the impacts of restoration treatments on songbird populations directly [21] and which habitat components influence such responses [22]. Having repeated measurements permitted analysis of consistency of songbird responses over time (compared to vegetation responses).…”

Effects of Vegetation and Background Noise on the Detection Process in Auditory Avian Point-Count Surveys

“…Most bird surveys are conducted under leaf-on conditions when birds are breeding, but they are also used to evaluate bird-habitat relationships following disturbances such as fire (Smucker et al 2005), Gypsy Moth (Lymantria dispar) infestation (Bell and Whitmore 1997), and silvicultural practices such as thinning (Hayes et al 2003). Each of these forms of disturbance produces conditions similar to our leaf-off habitats.…”