Bioacoustics Monitoring of Northern Spotted Owls and Barred Owls in Redwood National and State Parks

Hines, Terry; Schmidt, Kristin

Report to the U.S. Fish and Wildlife Service Interagency Agreement No. 4500036996 The invasion of the barred owl (Strix varia) during the past few decades in the Pacific Northwest and northern coastal California has resulted in a suppression effect on northern spotted owl (Strix caurina occidentalis; hereafter “spotted owl”) vocalization response rates (Crozier et al. 2006, Guitierrez et al. 2007, Livesey and Fleming 2007). In northern California, information (e.g., Diller 2013) indicates that, in cases in which spotted owl nesting territories have been usurped by barred owls, spotted owls may persist (in some cases for several years) in the vicinity of their former nesting site, despite not being detected during vocalization surveys for the species. Therefore, it is possible in some instances that while not responding to vocalizations broadcast by human surveyors, spotted owls: 1) are present while barred owls are present; 2) do occasionally vocalize; and 3) go undetected by human surveyors during the breeding season. Spotted owl detections have declined precipitously in Redwood National and State Parks (RNSP) in recent years. With a few exceptions, surveys of historic spotted owl territories have resulted in either no owl detections or barred owl only detections. The original spotted owl inventory work conducted in 1993 through 1995 identified between 37 and 40 spotted owl activity centers (ACs) in RNSP. Standard protocol (USFWS 2011) survey effort in 2013 and 2014, although highly reduced compared to 1995, resulted in 4 spotted owls detected at 3 separate sites. In contrast, there were 40 barred owl detections associated with 23 separate sites during this same time period. Barred owl ACs (similar to spotted owl ACs) have been documented within 1.0 mi of 36 historic spotted owl ACs; 14 of the 36 barred owl ACs were within 0.5 mile, and 11 were within 0.25 mi, of the spotted owl AC. Kelly et al. (2003) found that spotted owl occupancy in Oregon declined when barred owls moved to within 0.5 mi of a spotted owl AC, but acknowledged that in some cases spotted owls could still be present but not vocalizing when barred owls were also present. An automated bioacoustics recorder is a monitoring tool that does not require continuous human visitation of study sites and is capable of recording sounds at a site for an extended period of time with minimal human intervention. Automated bioacoustics recorders were placed in selected spotted owl territories with a history of barred owl occupancy, and where spotted owls have not responded in recent years of surveys in RNSP. Bioacoustics software allows users to 1) open and view recording files containing known vocalizations of species of interest; 2) manually find and mark (annotate) the known vocalizations; 3) generate a “recognizer”; and 4) automatically scan recorded files for likely matches (Wildlife Acoustics 2007a). The objectives of this pilot study were to: 1) determine if spotted owls are still present within historically occupied territories within RNSP, but are going undetected during standard protocol (USFWS 2011) surveys; and 2) characterize spotted owl calling patterns relative to barred owl calling, where the bioacoustics recorders revealed they were still present in their AC.