Forest Structure Within Barred Owl (Strix varia) Home Ranges in the Eastern Cascade Range, Washington

Abstract

Competitive interactions with Barred Owls (Strix varia) are an important factor contributing to the decline of the Northern Spotted Owl (Strix occidentalis caurina) population. Understanding the degree of similarity in fine-scale habitat associations for Spotted Owls and Barred Owls will help land managers evaluate whether there are specific vegetation conditions that could favor Spotted Owls over Barred Owls. From March 2004 to September 2006, I tracked 14 radio-tagged Barred Owls in the Okanogan-Wenatchee National Forest in the eastern Cascade Range, Washington. I analyzed forest structure characteristics from 170 plots sampled within areas used by the radio-tagged owls. I identified three forest types present within the Barred Owl home ranges, including: (1) open ponderosa pine (Pinus ponderosa), (2) simple-structure Douglas-fir (Pseudotsuga menziesii), and (3) complex-structure grand fir (Abies grandis). I compared individual forest structure characteristics and the three forest types to the intensity of Barred Owl use based on repeated measures of seasonal utilization distribution values at each plot using hierarchical mixed-effects models. Intensity of Barred Owl use during the breeding season was higher in areas with greater abundance of grand fir trees, taller and more diverse tree heights, more total trees per ha, more trees 12.7–22.9 cm dbh, more tree canopy >4.9 m, and less ground-cover vegetation <0.6 m. During the nonbreeding season, intensity of Barred Owl use was higher in areas with more trees 12.7–22.9 cm dbh, more total trees per ha, gentle slopes, and increased tree species diversity. Barred Owls used the structurally diverse grand fir forest type more intensively than the other two types during the breeding season. Intensity of use did not differ across the types during the nonbreeding season. Forest structure characteristics used by Barred Owls in this study were within the range of conditions reported to be used by Spotted Owls in the eastern Cascade Range.