Western screech-owl occupancy in the face of an invasive predator

Abstract

Invasive predators can alter ecosystem function and be detrimental to native wildlife by direct predation and through exploitative and interference competition. Barred owls (Strix varia), a native species in eastern North America, have expanded their range to Pacific Northwest forests, threatening native owls and community dynamics. The western screech-owl (Megascops kennicottii) is a species of conservation concern, and the apparent population decline coincides with the arrival of barred owls. We collected data in 2021 under a broad scale passive acoustic monitoring program at 2482 stations in nine study areas to evaluate western screech-owl distribution and potential threats to population persistence in the Pacific Northwest. We fitted single-species occupancy models to estimate barred owl and western screech-owl landscape use and identify important features of habitat. We quantified effects of barred owls on western screech-owl occupancy and detection probability using co-occurrence models. Barred owls used approximately 0.79 (95 % confidence interval: 0.72, 0.85) of our sampling locations. Western screech-owl occupancy and weekly detection varied by study area from 0.24 (0.15, 0.34) to 0.92 (0.89, 0.95) and 0.12 (0.08, 0.16) to 0.71 (0.69, 0.74), respectively. Western screech-owl detection dropped slightly from 0.36 (0.31, 0.41) to 0.29 (0.26, 0.33) where barred owls were present. Western screech-owls frequently occupied sites used by barred owls but occurred less in older forests where barred owls were present, suggesting they may experience higher predation risk in older forests. Western screech-owls may be unable to avoid habitat used by ubiquitous barred owls, but they may adapt behaviorally to avoid this invasive predator.