U.S. Department of Agriculture: Animal and Plant Health Inspection Service



Katie M. Dugger, United States Geological Survey
Alan B. Franklin, USDA APHIS WS NWRC
Damon B. Lesmeister, United States Forest Service
Raymond J. Davis, United States Forest Service
J. David Wiens, United States Geological Survey
Gary C. White, Colorado State University
James D. Nichols, University of Florida
James E. Hines, United States Geological Survey
Charles B. Yackulic, United States Geological Survey
Carl J. Schwarz, Simon Fraser University
Steven A. Ackers, Oregon State University
L. Steven Andrews, Oregon State University
Larissa L. Bailey, Colorado State University
Robin Bown, United States Fish and Wildlife Service
Jesse Burgher, United StatesForest Service
Kenneth P. Burnham, Colorado State University
Peter C. Carlson, Colorado State University
Tara Chestnut, United States National Park Service
Mary M. Conner, Utah State University
Krista E. Dilione, United States Geological Survey
Eric D. Forsman, United States Forest Service
Scott A. Gremel, United States National Park Service
Keith A. Hamm, Green Diamond Resources
Dale R. Herter, Raedeke Associates
J. Mark Higley, Hoopa Tribal Council
Rob B. Horn, United States Bureau of Land Management
Julianna M. Jenkins, United States Forest Service
William L. Kendall, United States Geological Survey
David W. Lapmphear, Green Diamond Resources
Christopher McCafferty, United States Forest Service
Trent L. McDonald, McDonald Data Sciences LLC
Janice A. Reid, United States Forest Service
Jeremy T. Rockweit, Oregon State University
David C. Simon, United States Geological Survey
Stan G. Sovern, Oregon State University
James K. Swingle, United States Forest Service
Heather Wise, United States Bureau of Land Management

Date of this Version



United States Geological Survey Open-File Report 2023–1012, 25 p.

doi: 10.3133/ ofr20231012

Prepared in cooperation with Oregon State University


United States government work


The northern spotted owl (Strix occidentalis caurina; hereinafter NSO) was listed as “threatened” under the Endangered Species Act in 1990 and population declines have continued since that listing. Given the species’ protected status, any proposed activities on Federal lands that might impact NSO require consultation with U.S. Fish and Wildlife Service and part of that consultation often includes surveys to determine presence and occupancy status of the species in the proposed activity area. The objective of this report is to present study-area specific estimates of the probability of detection for NSO pairs from twelve 2-week seasonal survey periods using data from a recent range-wide meta-analysis. These estimates were a by-product of pair occupancy modeling but might provide insight into potential changes in the effect of the invasive barred owl on NSO detection rates. We used two-species multi-season occupancy models to estimate the probability of detection for NSOs on each of 11 study areas for each 2-week survey period and relative to the range-wide effect of barred owl presence or absence. Detection probabilities within the season generally increased from the earliest surveys in March through mid-season, decreasing again in the late season on five study areas. For three other study areas, detection rates were highest during the earliest survey periods in late March or early April. Estimates of cumulative seasonal detection of NSO (across a maximum of six within-season surveys) were less than 0.90 when barred owls (BO) were present on all but one study area, regardless of when surveys were conducted within a season. However, despite low detection rates, the probability that a territory was occupied when an NSO pair was not detected over six within-season surveys was also very low. When BO are not present on a territory, a six-survey protocol had a high probability of detecting an NSO pair at least once during the season on all study areas, except for the very lowest per-survey estimates. Conducting most surveys earlier in the season, when the probability of detecting pairs is highest (through May on most areas) could improve seasonal detection rates. However, alternative methods of population monitoring—such as the use of passive acoustic recorders—may be needed to continue monitoring NSO for research and management.