Barred Owls and Northern Spotted Owls in the Eastern Cascade Range, Washington

Abstract

Competitive interactions with barred owls (Strix varia) are an important factor contributing to the observed decline in the northern spotted owl (Strix occidentallis caurina) population in Washington. My goal was to develop an understanding of barred owl habitat associations at spatial scales corresponding to home ranges and larger landscapes, and to apply that understanding to evaluate potential patterns of population interaction between spotted owls and barred owls in the eastern Cascade Range, Washington. I found that during the breeding season, barred owls used portions of their home ranges characterized as complex-structure mixed grand fir (Abies grandis) forest more than open ponderosa pine (Pinus ponderosa) or simple-structure Douglas-fir (Pseudotsuga menziesii). At a landscape scale, barred owls were associated with a broader range of forest structure and species composition conditions than spotted owls, but barred owls were more strongly associated with gentle slopes in lower topographic positions. Seventy-two percent of the areas used by spotted owls were located on slopes >16 degrees, and 72% of the areas used by barred owls were located on slopes <16 degrees. Overlap between barred owl and spotted owl habitat had a substantial effect on spotted owl pair site occupancy dynamics from 1989 to 2011. Site occupancy probability declined for all of the spotted owl pair sites, but it declined much less for sites with more good spotted owl habitat that overlapped with poor barred owl habitat within 500 ha of the spotted owl activity centers. Despite differences in space use and landscape associations displayed by the two species, displacement by barred owls had substantial detrimental impacts on spotted owl population performance in individual-based population modeling scenarios. The most plausible estimates of barred owl vital rates produced spotted owl population declines of approximately 60% to 80% relative to the spotted owl population estimate without barred owl displacement. Population simulations suggested that this landscape was capable of supporting at least twice as many barred owls as spotted owls, without considering the effects of inter-specific competition. When territorial displacement effects were incorporated, this landscape supported more than seven times as many barred owls as spotted owls.