Response of Passerine Birds to Forest Edge in Coast Redwood Forest Fragments

Abstract

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 × 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 categorized as edge species. Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), 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.