Researchers with Point Blue Conservation Science and the National Park Service will newly survey barred owls in the Point Reyes National Seashore and nine county parks this year, building upon two decades of monitoring northern spotted owls.

“Marin is really the last remaining area within the entire range of the northern spotted owl that has not been completely invaded by barred owls,” said Dave Press, a wildlife ecologist for the seashore. “It’s just a matter of time.”

The proliferation of barred owls in the western United States is threatening northern spotted owls across their range, from the North Bay to British Columbia. The larger and more aggressive barred owls are winning high-quality habitat, pushing out a species that has been listed as endangered since 1991. 

Despite the challenges facing northern spotted owls elsewhere, Marin supports a relatively strong population, and just a few barred owls have been detected in the area.

Marin County Parks has contracted Point Blue to survey northern spotted owls in Marin since 1999, but this is the first year that researchers will survey barred owls. The county agreed to pay $84,929 for the research, which also includes songbird monitoring at the Bolinas Lagoon. 

At the same time, $30,000 of park service funds awarded to the seashore will allow for more expansive barred owl monitoring for at least two years.

Point Blue and the park currently collaborate on spotted owl research, combining information from about 40 sites observed by the park service and another 50 sites monitored by Point Blue, to create a comprehensive database.

“Initiating this barred owl surveying is to get a baseline, or initial inventory, of how many barred owls are in these open space preserves,” said Renee Cormier, an avian ecologist for Point Blue. “Understanding how these species are doing is a good tool for land managers in making decisions.”

Barred owls were first detected in Marin in 2002, in Muir Woods. About a decade ago, researchers estimated that between four and seven barred owls lived in Marin—in Mill Valley, Muir Woods, the Olema Valley and Point Reyes Station. In areas where the barred owls were observed, spotted owls had a higher degree of nest failure and a lower likelihood of detection by surveyors, Mr. Press said.

According to the United States Fish and Wildlife Service, competition from barred owls is one of the two biggest threats to the continued survival of the northern spotted owl, along with habitat loss. 

Three or four barred owls can live within the territory of one northern spotted owl, and the barred owls hatch more fledglings each year. 

“The barred owls have the ability to completely swamp the spotted owl habitat,” Mr. Press said. “If there is a significant barred owl invasion, the only way to conserve the northern spotted owl in Marin would be to remove those owls.”

The fish and wildlife service is experimenting with the removal of barred owls in Oregon and Washington, where it has killed 2,435 barred owls in six years. So far, the reduction of barred owls has led to stabilized spotted owl populations; populations continue to decline in areas where no barred owls are removed. The study will continue until next year.

The interest in northern spotted owls in Marin began after the 1995 Mount Vision Fire. The park service knew the blaze had impacted the owls, but the extent was unknown. At the time, scientists were aware of only one owl pair at Indian Beach and another at Muir Woods, but a subsequent three-year inventory showed territorial pairs everywhere. Since the initial inventory, demographic monitoring has taken place during each breeding season, from March to July. 

The first measure in an owl survey is occupancy. Northern spotted owls tend to stay in the same area year after year, so researchers return to those spots to look for nests in the forest canopy. If they don’t observe an owl during the day, they can play a recording of a hoot at night and listen for the owl’s response. This year, researchers will also play the hoot of a barred owl.

“When we play a call, it’s basically like an intruder coming into their territory,” Ms. Cormier said. “So, they respond with a territorial call back to say, ‘Hey, this is my territory.’” 

The park service in the past went on two barred owl survey routes, playing its hoot from Olema to Bolinas and along the Sir Francis Drake Boulevard corridor. This year, surveyors will map a grid over the landscape and establish a few call points within each grid cell. This comprehensive method will allow for easy comparisons with surveys elsewhere, Mr. Press said.

One hypothesis as to why barred owls came West could also explain why the birds haven’t established themselves in Marin. 

When Europeans settled the Great Plains, they suppressed wildfires and brought new land management practices that allowed more trees. The new habitat facilitated a westward movement through land that had been impassable. Forests in Sonoma, Napa and Mendocino have populations of barred owls, yet the lack of forested stopping points between Marin and these areas makes for a difficult flight.