Automated Recordings in Marin Signal Hope for Northern Spotted Owls

By Science Communication Specialist Jessica Weinberg McClosky, San Francisco Bay Area Inventory & Monitoring Network

Both range-wide passive acoustic monitoring and local in-person surveys are showing that—at least for now—the spotted owls here in Marin County are doing remarkably well. Biologists spotted this nestling in May.

NPS / Point Reyes National Seashore Association / Aiko Goldston

June 2024 - For millennia, the hoots of northern spotted owls carried across mature forests of the Pacific Northwest. In recent decades, they started falling silent in many areas because of competition from bigger, bolder barred owls native to eastern North America. But both range-wide passive acoustic monitoring and local in-person surveys are showing that—at least for now—the spotted owls here in Marin County, California are doing remarkably well.

Each spring, teams from Golden Gate National Recreation Area and Point Reyes National Seashore take to the forests on foot to monitor owls in-person. Starting in March, they call the owls and note who calls back. They follow up throughout the breeding season to find nests and count nestlings. This season, the monitoring team found a normal, high number of spotted owls living in the territories they searched. They even added a new owl territory to the list when they found a male owl in a new area. So far, it’s not a banner breeding year, with five out of 19 nests failing, but nesting success for the owls is historically variable. The monitoring team found just two barred owls in different territories early in the season.

Each spring, teams from Golden Gate National Recreation Area and Point Reyes National Seashore take to the forests on foot to monitor owls in-person.

NPS / Environment for the Americas / Gavin Healy

Such in-person monitoring no longer makes sense in other parts of the northern spotted owl’s range. There just aren’t enough owls left to find. So in 2020, the multi-agency Northwest Forest Plan began transitioning to passive acoustic monitoring across the northern spotted owl’s range from Washington state to Marin County. This entails setting out thousands of autonomous recording units—or ARUs—for several weeks each spring. Each ARU is programmed to record for several hours continuously every day before sunrise and after sunset, plus a ten-minute sample from each hour throughout the day. Scientists then use artificial intelligence to identify and tally owl calls in the recordings. From there they can estimate species numbers and distributions. In Marin, National Park Service biologists started deploying ARUs in 2021. In 2023, the partner agencies reached full ARU deployment, completing the transition to acoustic monitoring.

Now, those 2023 monitoring results are in. In general, they show that there were more barred owl calls further north. This makes sense, since barred owls first expanded their range west through Canada, and then began moving south. And where there were more barred owl calls, the ARUs picked up fewer spotted owls. The results from Marin match those patterns, but even still, they stand out.

Passive acoustic monitoring for the Northwest Forest Plan entails setting out thousands of autonomous recording units—or ARUs—for several weeks each spring all across the northern spotted owl’s range. Each ARU is programmed to record for several hours continuously every day before sunrise and after sunset, plus a ten-minute sample from each hour throughout the day.

NPS / Environment for the Americas / Avani Fachon

The forests of Golden Gate, Muir Woods, and Point Reyes are tiny relative to other acoustic monitoring sites and the survey area as a whole. The 27 ARUs deployed here represent just 0.7 percent of the nearly 4,000-ARU survey effort. Yet they recorded an incredible 16,631 northern spotted owl calls representing 34 percent of the rangewide total. The corresponding barred owl data highlight how that was possible. ARU’s in Marin national parks picked up just 171 barred owl calls, or 0.03 percent of the more than half-million barred owl calls recorded over the whole monitoring area. Two other California survey areas had the second and third lowest total barred owl calls, both numbering in the 6,000s. In northern parts of the Washington Cascades, ARU’s picked up no spotted owl calls at all, but recorded barred owls almost everywhere.

This range-wide acoustic monitoring gives scientists the crucial ability to compare what is happening with the owls across sites. The 2023 results suggest that Marin County is unique in still having so many spotted owls and so few barred owls. As such, it’s a critical place for people and parks to continue supporting northern spotted owl conservation.

For more information

• San Francisco Bay Area Network Northern Spotted Owl Monitoring webpage
• Pacific Coast Science & Learning Center Northern Spotted Owls webpage
  • San Francisco Bay Area Northern Spotted Owls Blog
• Contact Wildlife Technician Taylor Ellis

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