TWS2021: Northern spotted owls face extirpation due to barred owls

A large-scale analysis shows that barred owls are the biggest factor impacting northern spotted owls in the Pacific Northwest. Researchers believe some northern spotted owl populations are likely to become extirpated in the next couple decades. Others may persist in much smaller numbers, making them vulnerable to catastrophic events, like wildfires.

Every five years, field biologists, statistician and managers analyze data on northern spotted owl demographics to determine the health of the populations. In research that will be presented at The Wildlife Society’s 2021 virtual annual conference, scientists used these analyses, including data dating back to 1993, to look at 11 study areas in the Pacific Northwest, covering 27,000 square kilometers and 6,000 banded spotted owls.

“These study areas encompass most of the geographic range the northern spotted owl (Strix occidentalis caurina) from Washington state into Northern California,” said Alan Franklin, a project leader at the USDA National Wildlife Research Center, in his presentation.

The analyses used mark-recapture surveys to predict occupancy in each region. They also incorporated demographic factors and other information that could impact the spotted owls, including disturbance and the growing presence of invasive barred owls (Strix varia).

The team found that while female spotted owl reproduction success varied throughout the study areas, barred owls decreased reproductive success. They also found negative effects of barred owls on recruitment, which was also affected by the reproductive success the previous year. Overall, the analyses showed a steep decline in northern spotted owl survival since 2012.

“This decline was strongly correlated with barred owl presence in northern spotted owl territories,” Franklin said. That included 6-9% declines in spotted owls annually in six of the study areas and 2-5% annually in the other five areas. “Almost all of the study areas have less than 35% of their population remaining since 1995,” he said. “Occupancy of northern spotted owl territories are really experiencing substantial declines on almost all those study areas as barred occupancy of these territories increases and this is fairly evident across almost all of the study areas.”

What’s most worrisome, Franklin said, is the strong positive effect of barred owls on the persistence of northern spotted owls in their territories. The data also showed northern spotted owls were unlikely to colonize territories where barred owls were present.

However, habitat components associated with spotted owls had a positive effect on them, which helped ameliorate the negative effects of barred owls. “This emphasizes the importance of maintaining northern spotted owl habitat, even in the face of another threat that causes them to decline on the landscape,” he said.

Conference attendees can visit office hours for this contributed paper on Friday, Nov. 5 from 4 p.m. to 5 p.m. to learn more and ask questions.