Barred owl habitat selection in West Coast forests

Over the past decade, northern barred owls (Strix varia) have invaded the Pacific Northwest and have contributed to population declines for northern spotted owls (Strix occidentalis caurina). The two species are known to hybridize and have also been documented displaying competitive and aggressive behaviors. Thus, multiple scientific publications and conservation documents have identified the barred owl as a threat to northern spotted owls.

In some forests of the Pacific Northwest, including some intensively managed forests, northern spotted owls appear to be maintaining their distributions in the face of competition from barred owls. Furthermore, barred owls have not successfully colonized some intensively managed forests. Thus, better understanding of barred owl habitat relationships can potentially inform conservation of the northern spotted owl.

Recently, NCASI completed a study to identify habitat characteristics associated with nocturnal habitat selection (i.e., primarily foraging) by barred owls across a variety of landscapes ranging from a state and national park to intensively managed forests. The investigators developed a general resource selection function that may be useful for evaluating the effects of barred owl presence on population viability of northern spotted owls.

Results from the study have been published in a paper by Larry L. Irwin, Dennis F. Rock, and Suzanne C. Rock that recently appeared in the Journal of Wildlife Management. The abstract follows.

“The invasion of the Pacific Northwest, USA by northern barred owls (Strix varia) is a conservation conundrum because it contributes to lingering declines in populations of northern spotted owls (Strix occidentalis caurina). We evaluated factors influencing nocturnal (i.e., foraging) habitat selection by northern barred owls using a repeated‐studies design and information‐theoretic methods across 3 Pacific Northwest study areas, each containing a broad range of forest and environmental conditions. We constructed discrete‐choice resource selection functions (RSF) based upon telemetry points linked to forest inventory plots and map‐based physical environmental metrics within home ranges of radio‐tagged barred owls at Chehalis, Washington (n = 16), Springfield, Oregon (n = 22), and Arcata, California (n = 15). A general RSF based upon pooling data across study areas suggested barred owls selectively hunted for prey in lower‐slope positions on southerly aspects often near streams at low elevations, and often within red alder (Alnus rubra) dominated stands or in moderately dense patches of medium‐ and large‐diameter coniferous trees close to patches containing nests. The relative probability of use decreased with increasing densities of small‐diameter trees, suggesting barred owls avoided clearcuts and young plantations. These general patterns were modified by study‐area variation in tree species composition and density. Study‐area‐specific factors that were associated positively with barred owl habitat selection included increasing basal area of western redcedar (Thuja plicata) and red alder at Chehalis and increasing densities of western redcedar and basal area of bigleaf maple (Acer macrophyllum) and western hemlock (Tsuga heterophylla) at Springfield. At Arcata, high densities of Douglas‐fir (Pseudotsuga menziesii) trees and increasing basal area of tanoak (Notholithocarpus densiflorus) were negatively associated with barred owl habitat selection. Seasonal patterns of habitat selection did not differ dramatically although model coefficients suggested selection for specific tree species was weaker in the non‐breeding season and barred owls did not seek topographic situations that provided thermo‐regulatory benefits.” The information may help inform conservation strategies for reducing competition between barred owls and northern spotted owls or perhaps in predicting colonization of new areas by barred owls.”

Reference 

Irwin, L.L., D.F. Rock, and S.C. Rock. 2018. Barred owl habitat selection in west coast forests. Journal of Wildlife Management 82(1): 202-216. https://doi.org/10.1002/jwmg.21339