While people fret over fluctuations in San Francisco’s Homo sapiens population, one species has been steadily gaining in the city for two decades: the great horned owl.
Dominik Mosur, a local naturalist who leads birding tours, recalls the excitement over the discovery of an owl’s nest in the Presidio. That was back in the early 2000s, when the owl population was scarce. “There wasn’t enough habitat,” says Mosur.
Yet around that time the Presidio Trust was on the verge of concentrated efforts to convert the former Army base into a natural area that would sustain many wildlife species.
The work in the city’s northwest corner set an example that city land management agencies like the Department of Recreation and Parks would replicate, says Matt Zlatunich, a retired firefighter and avid local birder. “It heightened an awareness of good practices,” he says.
This and other widespread restoration efforts in SF’s natural areas, including Mountain Lake Park, opened up the skies, or rather treetops, for an influx of forest-dwelling Bubo virginianus.
Today you can hear their signature stuttering call, “Hoo-h’hoo-hoo-hoo,” echoing throughout the city as dusk descends.
Urban adaptors
Great horned owls are now one of the most abundant owl species in North America, living in a wide range of habitats including forests, rural lands, and urban areas. They settle in to roost around January, often in nests built by crows, great blue herons, hawks, and even squirrels. (In 2019, a Presidio webcam captured a pair of great horned owls as they squatted with their eggs in a red-tailed hawk’s nest. The hawk fought them for it and lost.)
Sightings are common in the Presidio’s Tennessee Hollow and Mountain Lake Park, but the owls have fanned out across the city. Other popular spots include Fort Mason, Golden Gate Park, Lands End, Twin Peaks, and McLaren Park. Last year, there were an estimated 29 great horned owls in the city.
Pretty much any open space with a small amount of forested canopy can serve as a nesting ground for the Strigiformes — even tiny McCoppin Square Park behind the Taraval branch library in the Sunset District.
This wide range shows that the owls “are doing well and have adapted to human environments,” says Whitney Grover of the Golden Gate Bird Alliance (formerly SF’s chapter of the National Audubon Society).
But it’s been a two-way street. With major restoration efforts over three decades, San Francisco has also improved the habitat for them to survive.
This kicked off with the Presidio’s transition from a military base with landfill, rifle ranges, and an airfield in the early 1990s to a thriving natural area today with sand dune vegetation, coastal woodlands, tidal marshes, and serpentine grasslands.
Two decades later, the ambitious transformation of Glen Canyon began, as SF’s Recreation and Park Department dredged Islais Creek and cleared invasive plants. Twin Peaks, Strawberry Hill, Heron’s Head, and many other places have also gotten natural makeovers, helping create an ecosystem now recognized as a biodiversity hotspot.
This is all good for the great horned owl — and its varied diet that includes small critters like mice and doves as well as larger prey like skunks, rabbits, and even other raptors. Zlatunich says a major benefit of a healthy great horned owl population is “the ecological services they provide.” Namely, they keep the rat population in check. It doesn’t hurt that last year, California passed a new law severely limiting the use of anticoagulant rodenticides — rat poison — in wildlife areas after some owls died from the effects of eating toxic prey.
Great horned owls are highly territorial and tend to stay in one area, particularly if they’ve had breeding success. Nesting pairs will chase off competitors, and they’ll even push out their own young once they fledge. Mosur says the offspring can and do stay nearby, provided there is enough for all to eat. On average, an adult great horned owl eats the equivalent of two rats per day.
It’s pretty incredible how many there are now.
Whitney Grover, Golden Gate Bird Alliance
Finding these owls often depends on hearing them, with their signature hoots that start around dusk and continue into the night. Sometimes photographers setting up tripods to fix their lenses on a snag in a eucalyptus or juniper tree can be a clue to a nest location.
Still, Zlatunich hesitates to be too forthcoming about where to spot them: “We keep owl locations secret to avoid disturbance.” That hasn’t stopped birders and residents from trying to track them for the past several decades.
In cahoots
Counting bird populations is easy today with apps like iNaturalist and eBird. But both those services have drawbacks when charting historical populations.
The most reliable dataset to mark the progress or demise of many bird species actually comes from one of the oldest citizen science events.
Each winter, birders gather in cities across North America for “the Christmas bird count” between December 14 to January 5. Each location selects a single day in this period to count as many birds as possible by sight or sound. The census started in 1900 as an alternative to Christmas hunting expeditions — an early conservation campaign. It is conducted the same way every year for all species, with volunteers forming small groups to cover a “count circle” that’s 15 miles in diameter.
Locally, the Golden Gate Bird Alliance holds its count in three Bay Area cities, including San Francisco. It conveniently coincides with the start of the owl’s breeding season. Owls tend to be more vocal when they begin to pair up in December, says Grover, making them easy to “spot” for the purposes of the census.
The count tells the story of the great horned owl’s local resurgence. From the earliest recorded count in 1986 to 2003, each year turned up fewer than ten owls. From 2009 to the present, however, the city saw these numbers more than triple, from nine in 2009 to 29 in last December’s census.
“It’s pretty incredible how many there are now,” says Grover.
Zlatunich, who has participated in the Christmas census for more than 20 years, says the resurgence means more opportunities to catch mated pairs dueting to each other. “Hearing them at night is always kind of special,” he says.
As the city’s natural areas have become more habitable, there’s more opportunity for Zlatunich and others to experience that magic. Yet the site that prompted their resurgence faces new threats under the Trump administration, which started attempts in February to dismantle the Presidio Trust, which would likely hamstring its ecological work.
For now, the fight is stalled. Meanwhile the next generation of owls will hatch across the city, practicing their own songs under the cover of our urban canopy.
