Listening for Northern Spotted Owls in the Northwest's Parks

From the North Coast and Cascades Inventory and Monitoring Network, September 2023.

An autonomous recording unit in the Carbon River area of Mount Rainier. NPS photo / Galloway.

A new chapter

In Mount Rainier's Carbon River Rainforest in late September, technician Makenzie Groves scrambles over rain-soaked windfall, checking her GPS for a small recording unit — the newest tool for monitoring threatened northern spotted owls.

Over the past five years, autonomous recording units (ARUs) have become the front line of spotted owl monitoring. From the 1990s, teams at Olympic, Mount Rainier and North Cascades played calls to draw replies from territorial owls, banded birds and tracked nests and pairs year after year; some biologists learned to hoot well enough to get answers.

A biologist watches a female spotted owl in Olympic, 2012. NPS photo.

Why the owls are disappearing

• Olympic Peninsula: populations fell by more than 80% from 1995 to 2017.
• Mount Rainier: down nearly 75%; by 2017, just 18 territorial adults across more than 80,000 acres of habitat.

Logging got the owl listed as threatened in 1990, but inside the parks the main threat is a newcomer: the non-native barred owl, a voracious hunter that eats spotted owls' prey and much else.

Wildlife biologist Scott Gremel, who has studied Olympic's spotted owls since the 1990s. NPS photo.

"They eat a lot of insects. They eat salamanders. They eat everything. And they're certainly having a huge ecosystem impact," says Gremel. "We had an estimate of 230 pairs of spotted owls here in protected habitat—thought we were doing great. And now due to barred owls, they're almost gone."

Phase II: listening instead of calling

After the listing, the multi-agency Northwest Forest Plan began, covering 24 million acres and coordinating owl monitoring range-wide. In 2020 it moved to "phase II": as owls dwindled, callback surveys were phased out and ARUs deployed in the same areas, with two years of overlap. By 2023, every study area used passive recording.

Study areas monitored by passive acoustics in 2022. Ten 20% sample areas (red), all overlapping past owl study areas — Olympic Peninsula, Cle Elum, Mount Rainier, Oregon Coast Range, HJ Andrews, Tyee, Klamath, South Cascades, Northwest California and Marin County — and two 2% areas (blue), Umpqua and Gifford Pinchot national forests. All federal forests in the range are to be sampled at 2%. From the 2022 Bioacoustics Annual Report, USDA Pacific Northwest Research Station and NPS Pacific West Region.

How the recorders work

• Schedule: over a six-week deployment, four-hour sessions at dawn and dusk each day, plus 10-minute blocks every hour.
• Coverage: four ARUs in each 5 km² hexagon; random hexagons cover 20% of suitable habitat in the old callback areas.
• Analysis: at Oregon State University, a machine learning model trained on owl calls scans the audio. It also listens for 35 other species, including threatened marbled murrelets, and flags barred owls, opening questions about where the two species overlap.

One of 96 ARUs placed in Mount Rainier in 2022. NPS photo / Galloway.

The data flood: in 2022, Mount Rainier's 96 units recorded more than 50,000 hours; on the Olympic Peninsula, 474 units captured over 30 years' worth. Experts are still refining the model, and new questions arise: how many hoots mean a territory holds a nesting pair? How best to count the quieter females?

Crew members bike out the washed-out Carbon River Road with retrieved units. NPS photo / Galloway.

Still in trouble

That fall ended Mount Rainier's second year of acoustic monitoring; 70 miles northwest, Olympic finished its sixth. North Cascades deployed its first ARUs in 2023, at the lower intensity — one in 50 hexagons instead of one in five.

New methods can't change the facts: of Mount Rainier's 96 units in 2022, only four recorded spotted owls. As another season begins, a hard question looms: what does stewardship mean for a species facing an existential threat inside the parks themselves?

More: multi-agency owl monitoring, spotted owls at Mount Rainier, Washington's 2024 status update, a 2021 meta-analysis of Northwest spotted owl populations, and the 2022 bioacoustics report.

Sources

Based on "Dwindling Numbers Spur a New Approach to Northern Spotted Owl Monitoring," North Coast and Cascades Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. The page's sample spectrograms, from a journal article (Ruff and others, 2020), are not reproduced.