Why are peregrine falcon numbers declining again in the United States?

Something alarming is happening to peregrine falcons in North America. Their numbers are declining for the first time since the 1960s and 1970s, when the species was on the brink of extinction due to the widespread use of the pesticide DDT. Fortunately, a massive peregrine falcon recovery effort was launched in response, breeding thousands of captive-bred falcons and releasing them across North America. It was a spectacular success, with the birds’ numbers increasing nationally by more than 5% per year for decades. In 1999, the peregrine falcon was finally removed from the endangered species list, a remarkable achievement in the annals of conservation.

I am proud to say that I was part of the original cohort of raptor enthusiasts who helped save the peregrine falcon in the 1970s. None of us could have imagined that we would see this species decline again in our lifetimes. But sadly, more than half a century after those heady days, the peregrine falcon may once again be in danger.

I recently spoke with Kathy Clark, manager of New Jersey’s Endangered and Nongame Species Program, who told me that the peregrine falcons that nest in her state have had an unusually high adult replacement rate over the past two years. Healthy raptor populations always have a good supply of floaters—nonbreeding adults ready to step in if one or both birds in a nest die. But if the surplus dwindles, a species can run into trouble, as happened when DDT caused the birds to lay thin shells that often didn’t hatch. There simply weren’t enough extra falcons to replace the breeders when needed.

That seems to be the case now, and Clark knows it. These New Jersey birds are among the most closely monitored peregrine falcons in the world. Most nesting adults are banded, allowing researchers to know when a falcon disappears. In many other places, people can only count known peregrine falcon nests. An adult pair might be present, but there’s no way to tell if they’re the original pair or new arrivals.

“Of the 44 nesting peregrine falcons, half were missing this year,” Clark says. That’s a 50 percent replacement rate, but if you look at just the coastal areas, the rate is a whopping 63 percent. “Our normal replacement rate is about 15 percent,” she says.

Make no mistake: These extinct birds are dead. A hawk will not abandon its nesting territory. In addition to the adult turnover, two New Jersey nesting areas were unoccupied this year, and a third previously active nesting site was occupied by a first-year female and a second-year male. The nest failed. Young birds like this one rarely produce young as successful as older, more experienced adults.

“A lot of banded birds go missing and are replaced, usually by banded birds, which helps us know who they are,” Clark says. “In the past, there were always enough birds floating around that a missing bird would be replaced immediately.”

Virginia has a monitoring program very similar to New Jersey's, and most of the state's nesting peregrine falcons are banded. Bryan Watts, director of the Center for Conservation Biology at William and Mary College, has seen a similar increase in adult turnover on peregrine falcon ranges in Virginia.

“We’ve had some turnover in the past, but that wasn’t the case this year,” he said. “Here along the Atlantic coast, birds are typically replaced quickly, usually within a few days. But this year, we had two vacant territories on the coast and a few nests were holding second-year females, which is concerning. It suggests that the floating nest pool is depleted.”

What could be causing this phenomenon? Some have suggested that declines in prey species or contamination by neonicotinoid pesticides or flame retardants could be the cause. But others, including myself, think it is more likely that the dead falcons were infected with highly pathogenic avian influenza (HPAI), which has been affecting waterfowl, seabirds, and shorebirds in recent years. Interestingly, the highest turnover rates in New Jersey and Virginia occurred in coastal areas, where these species make up the majority of the peregrine falcon’s prey.

So far, the evidence is circumstantial but compelling. Two dead adult peregrine falcons have been found in nests in New Jersey. One tested positive for HPAI and the other was too decomposed to test, while in Virginia, two dead peregrine falcons have tested positive this year. According to the USDA, bird flu has been confirmed in more than 50 dead peregrine falcons nationwide since 2023, but that number is likely far below the true figure. After a falcon feeds on a sick bird, the raptor typically flies to a cliff or other remote perch and eventually dies there within a few days. It doesn’t leave behind a body that’s easily accessible for testing.

This problem isn’t limited to the East Coast. I recently spoke with Skip Ambrose, who began monitoring peregrine falcons on the upper Yukon River in Alaska for the U.S. Fish & Wildlife Service in 1973 and has continued to do so, even though he’s been retired for a decade. He and his wife, Chris Florian, now have their own boat and travel a 165-mile stretch of the river each year in late May and June to determine nesting site occupancy, and again in July to count the number of young falcons produced by the nests. He remembers well the low point in 1973, when only 12 peregrine falcon sites in his study area were occupied. In 2012, he counted more than 60 pairs. But during the 2023 and 2024 breeding seasons, the numbers have clearly declined.

“We just needed adults,” he said. “We had a lot of single people.”

By 2024, a third of the nesting sites he visited were unoccupied, and some had only a single falcon present, with no signs of breeding. “The fact that we see single birds on nesting sites means the floating population must be very small. When something happens to an adult, there aren’t enough replacements to replace it.”

Ambrose speculates that avian flu is at the root of the problem. “I think it’s too much of a coincidence that peregrine falcon numbers are declining after the highly pathogenic avian flu,” he says. “Peregrine falcons are the perfect machine for searching out sick birds.” And it’s true: They always seek out weak, easy-to-catch individuals. This is a good thing ecologically, because it promotes survival and reproduction of the fittest. But if the prey they consume has a deadly infectious disease, the falcon usually dies.

I’ve also heard from researchers on the West Coast, like Bud Anderson, director of the Falcon Research Group in Washington state. “Peregrine falcons in the San Juan Islands have had a catastrophic decline,” he says. “We used to see 16 to 18 pairs there, but this year we’ve only found five active areas, and only two of them have produced young.”

Interestingly, peregrine falcons in eastern Washington and cities do not appear to be affected yet. Could it be their diet? Anderson thinks so. Birds in eastern Washington and urban areas do not consume as many waterfowl and shorebirds as coastal birds. Avian influenza has caused massive die-offs of waterfowl in the Puget Sound region over the past two years. So far, seven dead peregrine falcons have tested positive in Washington state.

“Puget Sound is home to tens of thousands of waterfowl each winter,” he said. “We know that peregrine falcons that nest in the San Juan Islands feed on a lot of waterfowl, but that’s less the case in eastern Washington and in cities.”

California is also affected. Grainger Hunt is a raptor researcher who has worked with the Peregrine Fund for many years. He has been monitoring five peregrine falcon nests near his home in northern California for 22 years.

“This year, four out of five nests failed and we had our first subadult falcon in a nest,” he told me. “I think the birds die during incubation. That’s when the shorebirds come in on migration.”

Hunt also believes that highly pathogenic avian influenza is to blame. “It’s just a coincidence, and it seems more deadly and more persistent than anything we’ve seen so far,” he says. “At the rate things are going, birds could disappear from some areas.”

Many questions remain. Is avian influenza really the cause of the decline in peregrine falcon populations, or is it something else? And if it is highly pathogenic avian influenza, will it simply sweep through the bird population like a black wave and disappear, leaving the birds to recover? Or will it continue its deadly spread, now infecting birds around the world, and decimate peregrine falcon populations to the point where they will need to be saved again? It is too early to tell, but those who know the species best are deeply concerned, and that should be a wake-up call to everyone.

Tim Gallagher is a writer based in Freeville, New York, and the author of Hawk Fever, The Grail Bird, Imperial Dreamsand three other books.