Even the fastest bird on Earth can’t outfly bird flu

The high-speed hunters that clawed their way back from near extinction after the DDT era are now declining again — this time in step with a deadly strain of avian influenza sweeping California’s coast and many other parts of the country. Research that is currently under review found peregrine falcon populations have fallen by 65% in just three years. 

“This decline rate is at least as deep as it was for DDT,” lead researcher Zeka Glucs, director of the UC Santa Cruz Predatory Bird Research Group (PBRG), said of the insecticide dichlorodiphenyltrichloroethane. She emphasized that this is “not only a local issue. This has happened globally.”

The timing of the die-off aligns with the arrival of highly pathogenic avian influenza, commonly known as bird flu, first detected in California wildlife in the winter of 2022. After decades spent documenting peregrine falcons’ dramatic recovery, researchers are calling for expanded long-term monitoring.

The comeback kids of the cliffs

Peregrine falcons are no strangers to past trauma. In the 1960s, the species nearly vanished across North America due to DDT, which thinned eggshells and decimated breeding populations. Beginning in the 1970s, falconers and wildlife biologists launched a massive recovery effort, breeding peregrines in captivity and releasing them along California’s coast and in urban areas where tall buildings mimic cliffs.

Pioneered by the PBRG and The Peregrine Fund, the effort succeeded. By 1999, peregrines were removed from the federal endangered species list, and California delisted them in 2006. Today, the birds remain protected under laws such as the Migratory Bird Treaty Act and can be found nesting on coastal cliffs, canyon walls, bridges and skyscrapers across the state.

“Peregrine falcons were that story that we learned about in school, about how to recover endangered species and the kind of heroic efforts that it took to bring peregrine falcons back after DDT,” Glucs said.

Life on the ledge

For decades, the UCSC group has monitored peregrine nests from Point Reyes National Seashore in Marin County south to Salinas, and as far east as Sacramento. Scientists, students and volunteers track whether known nests are occupied by a pair of falcons, a single bird or left empty.

Determining that a territory is vacant requires patience and a lot of snacks, said UCSC PBRG intern Natalie Courtney. Observers hike into the wilderness to reach vantage points overlooking cliffs or canyon walls, within spying distance of rocky nesting areas. There, they scan and listen for peregrines for four hours at a time. If no birds appear, the site is considered vacant. Much of this work relies on volunteers who return to the same nesting sites year after year, learning “where the birds hang out” and even that “each bird has its own personality,” with some being especially “feisty,” said Courtney.

Research and volunteers visit each nest at least twice during the breeding season, which typically spans from March to June. In addition to these visits, the research group conducts bird banding and collects biological samples, such as feathers. Nest camera footage and volunteer observations through the California Peregrine Project further enrich this data, creating a comprehensive record of peregrine behavior across the region.

Top of the food chain — bottom line for ecosystems

Peregrines are fiercely territorial during breeding season, with a single pair often claiming the same nest year after year. Normally, if one bird disappears, another moves in quickly. That predictability is what made the recent pattern so alarming.

For more than two decades, most nests remained occupied. Beginning in the winter of 2022, occupancy rates dropped sharply, aligning with the spread of bird flu among California wildlife. By 2025, breeding pairs occupied roughly two-thirds fewer territories. “When those started to go dark, I started to be very concerned and that is what led to this publication,” Glucs said.

The California Department of Fish and Wildlife and the U.S. Department of Agriculture confirmed 27 cases of highly pathogenic avian influenza (HPI) in peregrine falcons within the areas Glucs’ team monitored for the study. “One — on camera — fell off the nest, was brought to a rehab center, tested positive for HPI and died soon after,” she said.

Peregrines may be especially vulnerable to bird flu because of the prey they hunt and eat. They capture birds in midair using high-speed acrobatic dives called “stoops.” Along California’s coast, their prey often includes gulls, terns, sandpipers and ducks — species heavily affected by avian influenza. Infected birds are easier to catch, creating a pathway for the virus to move up the food chain. As top-tier predators, peregrines can serve as indicators of broader ecological health.

Adding to their vulnerability, California’s breeding peregrines are largely nonmigratory, meaning most stay near the same territories year-round. Without a steady influx of birds from elsewhere, researchers cannot predict how quickly empty nests might be refilled.

Resilience in the crosswinds

The recent decline does not necessarily signal another population collapse, but it is a stark reminder that even recovered species remain vulnerable. Kelly Sorenson, executive director of the Ventana Wildlife Society, explained that wildlife biologists often assess recovery using three factors: representation, the genetic diversity within a species; redundancy, the presence of multiple populations across different regions; and resiliency, the ability of populations to withstand new threats. Peregrines appear strong in the first two, but the current outbreak of avian influenza could test their resiliency.

Sorenson, who has been deeply involved in efforts to restore California condors to the wild, said bird flu is a challenge all vulnerable wildlife must face. “It’s here now. It’s endemic. It’s not going away,” he said. 

In response to recent outbreaks of highly pathogenic avian influenza, many free‑ranging condors in central California have been vaccinated under emergency authorization. The vaccine, developed with federal researchers and the animal‑health company Zoetis, is still considered experimental and is currently used mainly for endangered or managed birds under close veterinary supervision, so it’s not yet an option for peregrine falcons.

Glucs draw lessons from the falcon’s successful recovery history to guide the team’s next steps. There was once a tightly knit network across the state that guided peregrine recovery amid the DDT onslaught. That coordinated monitoring has since fractured into “siloed” regional groups, but she hopes to reconnect them. “The only reason we were able to detect this decline was because there was a long-term monitoring program in place,” she said. “I’m just really grateful to the networks that have been looking at peregrine falcons and collecting this data year after year.”

The coming years will reveal whether one of conservation’s greatest comeback stories can survive yet another unexpected threat.

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