Barred owl habitat selection in west coast forests. Issue 1 (4th October 2017) - Irwin, Larry L.
ABSTRACT: 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. © 2017 The Wildlife Society. Abstract : Barred owl foraging in 3 Pacific Northwest, USA study areas was associated with patches that contained medium‐ to large‐diameter trees, often in lower slope positions near streamside zones containing red alder trees. Our analysis has utility for forest conservation planning, accounting for barred owls in habitat‐based population viability analyses for northern spotted owls, and possibly predicting range expansion of barred owls.