No 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 Pacific‐slope 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.

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.

Although benefits of interdisciplinary studies are numerous, potential exists for data acquisition for some aspects of such studies to impact data acquisition for other aspects. This may be particularly true in studies involving both trapping of small mammals and assessment of bird populations. We summarize the incidence of birds captured during 8 research projects in Massachusetts, Oregon, and Washington that used 5 types of small‐mammal traps, and discuss possible impacts of small‐mammal trapping on bird surveys and possible mitigation measures. In these studies, 867 birds representing 17 species were captured in 703, 138 total trap‐nights (TN). The song sparrow (Melospiza melodia), spotted towhee (Pipilo maculatus), Steller's jay (Cyanocitta stelleri), and gray jay (Perisoreus canadensis) accounted for 86% of all bird captures; ground‐foraging species accounted for 54% of all bird captures. Relatively high capture rates were observed in Sherman (H. B. Sherman Traps, Inc., Tallahassee, Flor.) and Tomahawk traps (Tomahawk Live Trap Co., Tomahawk, Wisc.) (4.1 and 9.8 birds/1,000 TN, respectively) in study 1, whereas pitfall and Ugglan (Grahnab, Ekhaga Marieholm, Hillerstorp, Sweden) traps had negligible captures (<0.1 and 0.0 birds/1,000 TN, respectively) in 4 studies that used them. On 11 occasions capture rates in 1 stand on 1 day in study 1 ranged from 50.0–100.0 birds/1,000 TN. Despite relatively high capture rates of birds in Tomahawk and Sherman traps in 2 studies, the 6 other studies had limited avian captures in both these and all other trap types used. Variability in avian capture rates within trap types across studies makes it difficult to predict the time of year when and habitats where avian captures could occur. Therefore, researchers should be mindful of potential negative impacts of small‐mammal trapping on avian aspects of research when designing interdisciplinary studies that include both avian and small‐mammal components conducted simultaneously at the same sites.