Center for Conservation Biology

Improving survey techniques of Northern spotted owl

Research overview

Close up image of a northern spotted owl sitting on a tree branch in daylight

Shasta-Trinity National Forest, California
Years surveyed: 2009-2011
Related publication: Using Detection Dogs to Conduct Simultaneous Surveys of Northern Spotted (Strix occidentalis caurina) and Barred Owls (Strix varia).)Ryan Hepler

The northern spotted owl, Strix occidentalis caurina, is the flagship threatened species of the Pacific Northwest. Federally listed under the Endangered Species Act in 1990, the northern spotted owl continues to decline at a rate of about 4% throughout its range. Despite the fact that the northern spotted owl is one of the best-studied wild vertebrate species in the world, the relative importance of the threats that it faces remain controversial.

Without question, one of the most serious threats facing the northern spotted owl is the recent range expansion of another closely related owl species, the barred owl, Strix varia. Because barred owls may attack and kill northern spotted owls, northern spotted owls are known to vocalize less when around barred owls. This poses a serious problem for the manager whose primary means of establishing northern spotted owl presence is the owls’ vocal response to simulated calls.

Large dog looks at camera, while standing in a heavily forested area with a spotted owl on a branch in the background

Conservation Canines, scat detection dog, Max with a northern spotted owl.

When repeated vocalization surveys yield no northern spotted owl vocal response for three consecutive years, the territory is considered unoccupied and habitat protection is lifted. However, vocalization survey results may be unreliable if northern spotted owl are unlikely to vocalize due to barred owl presence. To address this problem we developed a new survey technique using Conservation Canines, scat detection dogs trained to locate owl pellets by scent.

We compared success of detection dog surveys to vocalization surveys of seventeen 2 km x 2km polygons using a slightly modified version of the U.S. Fish and Wildlife Service’s Draft 2010 Vocalization Survey Protocol.

Data chart showing that using dogs to detect barred owls is more effective than vocal calls.

Barred Owl Detection during the 2010 field season.

Pellets were DNA confirmed to species, whereas vocalization surveys were confirmed by visual identification of the owls. Occupancy models were used to calculate northern spotted owl and barred owl detection probabilities for both survey methods (Figure 1a and 1b).

Data chart showing that dogs detected more spotted owls than the vocal method

Detection rates of northern spotted owl during the 2010 field season. “Circles” represent detection dog surveys, “squares” represent vocalization surveys. Click image to enlarge.


Wasser, Samuel K, Lisa S. Hayward, Jennifer Hartman, Rebecca K. Booth, Kristin Broms, Jodi Berg, Elizabeth Seely, Lyle Lewis, Heath Smith. 2012. Using Detection Dogs to Conduct Simultaneous Surveys of Northern Spotted (Strix occidentalis caurina) and Barred Owls (Strix varia). PLoS ONE 7(8): e42892.