Certhia

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.

Abstract: The response of passerine birds to forest edge was examined in old-growth and mature second-growth coast redwood (Sequoia sempivirens) forest in northern California. The study objectives were to determine which common passerine species are sensitive to edges during the breeding season and to estimate edge width for forest interior species. Response to edge was measured along twelve 100 x 400 m plots extending from the edge into the forest interior to obtain relative density of birds. Plots were surveyed 4 to 5 times in 1996 and 8 to10 times in 1997. We found that 14 common passerines showed a gradient of edge sensitivity. Steller's Jay (Cyanocitta stelleri) and Swainson's Thrush (Catharus ustulatus) had higher relative densities near edges than in the forest interior (P < 0.05) and were cat- egorized as edge species. Brown Creeper (Certhia americana), Winter Wren (Troglodytes trog- Iodytes), Pacific-slope Flycatcher (Empidonax difficilis), and Varied Thrush (Ixoreus naevius) had lower relative densities near edges (P < 0.05) and were categorized as interior birds. Based on exponential regression models, estimated edge widths were 140 m for Varied Thrushes, 85 m for Brown Creepers, 120 m for Winter Wrens, and 125 m for Pacific-slope Flycatchers. Creation of edges would probably benefit Steller's Jays (which may be a nest predator), may not benefit Swainson's Thrushes, and may be detrimental to species that avoid edges. We recommend that edge effects be taken into consideration when planning for the conservation of bird species in the region. Received 18 January 2000, accepted 23 January 2001.