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Passive Acoustic Monitoring Effectively Detects Northern Spotted Owls and Barred Owls

Northern spotted owl.
Scientists find a safer, less costly, and noninvasive method for monitoring northern spotted owl. Strategically placed recording units detected calls of northern spotted owls and barred owls over a range of forest conditions. These findings are being used to design and inform the transition from mark-recapture to passive bioacoustics as the primary monitoring method for northern spotted owl populations under the Northwest Forest Plan effectiveness monitoring program.
Fiscal Year
Research Unit(s)
Ecological Process and Function
Principal Investigator(s)
Northern spotted owls (Strix occidentalis caurina) are listed as threatened under the Endangered Species Act. Populations have been monitored since the mid-1980s by using labor intensive mark–recapture methods that require call-back surveys and using mice to lure owls for capture and leg-band reading. Land managers, conservationists, and researchers have sought alternative methods that yield robust data while being less costly, suitable for multiple species, safer for field crews, and noninvasive for spotted owls. To evaluate passive bioacoustics as an alternative survey method, researchers used autonomous recording units to detect calls of northern spotted owls and barred owls (Strix varia), a species that has expanded its range into the Pacific Northwest and threatens spotted owl persistence. They found that even in a study area with low occupancy rates on historical territories the probability of detecting a northern spotted owl exceeded 0.95 after 3 weeks of recording, and a probability near 1 for barred owls. These results demonstrate that passive bioacoustics can effectively meet the monitoring objectives of land managers, conservationists, and researchers, even in a landscape with high barred owl density. Because of these findings and those of other research efforts, passive bioacoustics have proven effective and will now be the primary method to monitor northern spotted owl populations under the Northwest Forest Plan effectiveness monitoring program. These findings are also foundational to a new protocol for pre-project and project monitoring that is in development by a working group led the U.S. Fish and Wildlife Service for project consultations.
Forest Service Partners
  • Pacific Northwest Region
External Partners
  • U.S. Bureau of Land Management
  • U.S. Geological Survey
  • U.S. Fish and Wildlife Service
  • National Park Service
  • Oregon State University