How a surprising San Jose spot became a last refuge for the Bay Area’s burrowing owl population

Near the huge sewage plant that treats San Jose’s wastewater at the southeast tip of the San Francisco Bay, wildlife biologist Phillip Higgins peers through binoculars. A powerline hums in the background while he scours the ankle-high grass ahead for signs of life.

In less than a minute, he spots what he is looking for — a small head with large yellow eyes is poking out of a buried pipe. This is a burrowing owl, less than a foot tall and weighing just ounces. Higgins indicates that if we approach, it will surely fly away.

As one of the Bay Area’s most imperiled birds, the owl has every reason to be skittish.

Sandwiched between office buildings and the sewage plant, this 200-acre slice of land is home to some of the last burrowing owls in the Bay Area. They were once common, but urbanization has paved over most of their grassland habitat. In October, the state designated the owls as a candidate for protection under the California Endangered Species Act.

But while burrowing owls are disappearing throughout the state, a collaboration between biologists and the City of San Jose has helped them maintain a talon hold on this spot. The scientists are now working to protect the few that remain in the Bay Area and reintroduce them to better habitats farther south. It is part of an effort led by the Santa Clara Valley Habitat Agency that seeks to bring the birds back from the brink.

When Higgins first started studying the owls at this site more than a decade ago, their future was uncertain. “We did our first surveys in 2012, and we only observed two burrowing owls,” he said. This year, Higgins and his colleagues counted 11 breeding pairs.

While that number might seem small, it is one of the largest groups left in the region.

Burrowing owls were historically most abundant at Warm Springs, San Jose International Airport and Moffett Airfield. All three had over 100 owls in the 1980s. This year, Warm Springs and the airport had no breeding owls, Higgins said. Moffett Airfield had just one pair.

According to the nonprofit San Francisco Bay Bird Observatory, ground squirrel eradication is one reason for the decline. Ground squirrels dig holes that burrowing owls appropriate. The owls cannot build their own homes.

“Ground squirrels have been considered a pest species for a hundred years and have been poisoned, shot and gassed,” said Sandra Menzel, director-at-large of Talon Ecological, a conservation nonprofit that Higgins co-founded. “But the burrowing owl needs ground squirrels to survive.”

In 2013, Higgins wrote a management plan that recommended increasing the ground squirrel population near the wastewater plant. This proved challenging because the site is flood-prone. During winter storms, water collects in the low areas, destroying burrows and drowning the small mammals. So in 2013, the city began to transport dirt to the site to raise some sections above the floodwaters. By 2015, they had added 80,000 cubic yards.

The soil transplant worked. The number of adult burrowing owls in the area reached a high of 36 in 2017, but the rainy winter that followed flooded the area for five months, killing owls and ground squirrels. The dirt mounds became islands where Higgins saw many animals taking refuge, and the survivors of the flood are now repopulating the site.

But even with this assistance, ground squirrel burrows are in short supply. Talon Ecological has created artificial burrows using six-inch diameter pipes connected to a box buried underground. The owls use these, but they are not as good as the real thing.

“Burrowing owls are very messy,” Menzel said. “The ground squirrel at some point goes in again and makes it all neat. It’s like their little room service.”

Landscaping, however, is provided by human volunteers. Burrowing owls like to stand watch outside their homes. If tall grass obstructs their view, they might leave and look for a new home. Grazing animals historically did the job. But in this small island of habitat, groups of biologists and volunteers mow, mulch and hand-weed to keep the vegetation to the owls’ liking. This is one of the only times the owls see people, as the site is closed to the public.

Squirrels and owls are not the only ones benefiting from this conservation work. From 2018 to 2022, Higgins and Menzel observed a pair of golden eagles nesting nearby. “It’s the first time in over 100 years that golden eagles have nested on the valley floor of San Jose,” Higgins said. He suspects the abundant prey drew them here. After a windstorm sent pieces of their nest to the ground, he and Menzel found 31 ground squirrel carcasses.

This conservation haven owes its existence to the sewage plant’s toxic history. The open space originally served as a buffer zone separating local communities from the plant, which at the time treated water with chlorine. If any escaped, it could shroud the area in poisonous gas.

The wastewater treatment plant no longer uses chlorine, but the City of San Jose has placed the site in a conservation easement to protect it from development. “The city has been very supportive,” Higgins said.

Indeed, if a burrowing owl visits the wastewater plant, it will temporarily suspend operations. “It’s like a security zone,” said Jennifer Voccola Brown, sustainability and compliance division manager for the City of San Jose.

Even with this support, the burrowing owls’ future here is far from certain. Surrounded by a city, there is nowhere for them to expand. In a small, isolated population, inbreeding becomes a concern.

Higgins has adopted the role of matchmaker. He sends blood samples to Colorado State University where they sequence the birds’ DNA. The results show how related the owls are to each other. Higgins uses this information to decide which individuals to pair up to maximize the population’s genetic diversity.

He then keeps them in an enclosure with an artificial burrow until they lay eggs. In this honeymoon suite, there’s not enough space to forage, so the team at Talon Ecological brings them mice — a rare treat for a species that mostly feeds on earwigs and other insects.

The fed owls produce more and healthier chicks. This is good news for efforts to ensure the species’ long-term survival. Talon Ecological is working with the Santa Clara Valley Habitat Agency to re-introduce them to Coyote Valley, near the city of Morgan Hill. Thanks in part due to supplemental feeding, all 18 birds the team released in 2023 successfully raised chicks.

While burrowing owls that migrate from Oregon and Washington occasionally visit Coyote Valley in the winter, none had bred there for many years. Higgins is not entirely sure why they disappeared.

Still, it seems like an ideal environment for burrowing owls. “There’s more open space, so those populations can naturally expand,” Higgins said.