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Something disturbing is happening to Peregrine Falcons in North America. Their numbers are heading downward for the first time since the 1960s and ’70s, when the birds were teetering at the edge of extinction due to the widespread use of the pesticide DDT. Thankfully, a massive Peregrine Falcon recovery effort was launched in response, producing thousands of captive-bred falcons and releasing them across North America. It was a spectacular success, with the birds' numbers increasing nationwide by more than 5 percent a 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’m proud to say I was part of the original cohort of raptor enthusiasts who helped save Peregrine Falcons in the 1970s. None of us ever imagined we would ever see this species declining in numbers again in our lifetime. But sadly, more than half a century since those heady days, the Peregrine Falcon might be in trouble once more. 

I recently spoke with Kathy Clark, chief of New Jersey’s Endangered and Nongame Species Program, who told me that the Peregrines nesting in her state have had an abnormally high adult replacement rate for the past couple of years. Healthy raptor populations always have a good supply of floaters—non-breeding adults that are ready to step in if one or both birds at an eyrie die. But if the surplus dwindles, a species can run into problems, which is what happened when DDT caused the birds to lay thin shells that often failed to hatch. There were just not enough extra falcons to replace the breeders when needed.

This seems to be happening now, and Clark would know. These New Jersey birds are some of the best-monitored Peregrine Falcons in the world. Most of the nesting adults are banded, so researchers know when a falcon goes missing. In many other places, people can only do occupancy counts at known Peregrine nests. A pair of adults might be present, but there is no way of knowing if it is the original pair or some new arrivals.

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

Make no mistake: Those missing birds are dead. A falcon will not abandon its nest territory. In addition to the turnover of adults, two New Jersey eyries were unoccupied this year, and a third previously active nest site was occupied by a first-year female and a second-year male. The nest failed. Younger birds like this are rarely as successful at producing young as older, more experienced adults.

“A lot of the banded birds are vanishing and being replaced, usually by birds that are also banded, so we know who they are,” Clark says. “In the past, there were always enough floaters that a missing bird would be replaced right away.”

Virginia has a monitoring program much like New Jersey’s, and most of the state’s nesting Peregrines are banded. Bryan Watts, director of the Center for Conservation Biology at William and Mary College, has noticed a similar spike in the adult turnover rate at Peregrine eyries in Virginia.

“We’ve had some turnover in the past, but it was nothing like this year,” he says. “Here along the Atlantic Coast, the birds are usually replaced quickly, typically within days. But this year we had two vacant territories on the coast and a couple of nests had second-year females, which is a concern. This suggests that the floater pool is stretched thin.”

What could be causing this? Some have suggested that a decline in prey species or contamination by neonicotinoid pesticides or flame retardants might be to blame. But others, including myself, think a likelier possibility is that the dead falcons became infected by Highly Pathogenic Avian Influenza (HPAI), which has been hammering waterfowl, seabirds, and shorebirds for the past few years. It’s interesting to note that the worst turnover rates in New Jersey and Virginia occurred in coastal areas, where those species make up more of the Peregrine’s prey base.

So far, the evidence is circumstantial but compelling. Two dead adult peregrines have been recovered at nests in New Jersey. One tested positive for HPAI and the other was too decomposed for testing, while in Virginia two dead peregrines tested positive this year. According to the USDA, avian influenza has been confirmed in more than 50 dead Peregrines nationwide since 2023—but this is certainly much lower than the actual figure.  After a falcon feeds on a sick bird, the raptor usually just flies off to a cliff or other distant perch and ends up dying there within a couple of days. It doesn’t leave behind an easily accessible corpse to test.

This is not just an East Coast problem. I recently spoke with Skip Ambrose, who began monitoring the Peregrines on the upper Yukon River of Alaska for the U.S. Fish & Wildlife Service in 1973 and has continued ever since—even though he’s been retired for a decade. He and his wife, Chris Florian, have their own boat now and cruise a 165-mile stretch of the river each year in late May and June to determine nest-site occupancy, and again in July to count the number of young falcons the nests produced. He remembers well the low point in 1973 when only 12 Peregrine Falcon eyries in his study area were occupied. By 2012, he counted more than 60 pairs. But in the 2023 and 2024 breeding seasons, the numbers were clearly falling.

“Adults were just missing,” he says. “We had a bunch of singles.”

In 2024, a third of the nest sites he visited were unoccupied and some only had one falcon present, with no sign of breeding. “The fact that we’re seeing single birds at nest sites means that the floating population must be way down. When something happens to an adult, there aren’t enough replacements to fill in.”

Ambrose speculates that avian influenza is to blame. “I think it’s too big of a coincidence that Peregrine numbers are dropping after the arrival of HPAI,” he says. “Peregrines are the perfect go-look-for-sick-birds machine.” And it’s true—they’re always seeking weak, easy-to-capture individuals. This is great ecologically, promoting the survival and reproduction of the fittest. But if the prey they consume is suffering from a deadly infectious disease, the falcon usually dies.

I’ve also heard from researchers on the West Coast, such as Bud Anderson, director of the Falcon Research Group in Washington State. “The Peregrines in the San Juan Islands have experienced a calamitous drop,” he says. “We used to see 16 to 18 pairs there, but this year we only located five active eyries, and only two of them produced young.”

Interestingly, the Peregrines in the eastern part of Washington State as well as in cities do not seem to be affected yet. Could it be their diet? Anderson thinks so. The birds in the eastern part of the state and urban areas don’t consume as many waterfowl and shorebirds as the coastal birds. There have been massive waterfowl die-offs from avian influenza in the Puget Sound region in the last two years. So far, seven dead Peregrines have tested positive in Washington State.

“Puget Sound supports tens of thousands of waterfowl each winter,” he says. “We know that the Peregrines nesting in the San Juans feed on huge numbers of waterfowl, but this is less so in eastern Washington and in the cities.”

California is also being affected. Grainger Hunt is a career raptor researcher who worked with the Peregrine Fund for many years. He has been checking five local Peregrine eyries near his home in northern California for 22 years.

“This year, four out of the five nests failed, and we had our first subadult falcon at one nest,” he tells me. “I believe the birds are dying during incubation. That’s when the shorebirds are coming through on migration.”

Hunt also believes HPAI is to blame. “It’s just too coincidental, and it seems more deadly and persistent than anything we’ve seen before,” he says. “At the rate this is going, the birds might be extirpated from some areas.”

There are still many questions at this point. Is avian influenza indeed causing the decline in Peregrine numbers, or is it something else? And if it is HPAI, will it just sweep through the bird’s population like a dark wave and then fade away, leaving the birds to bounce back? Or will it continue its deadly spread, which is now infecting birds worldwide, and decimate Peregrine populations to the extent that we'll need to save them again? It's too early to tell, but those who know the species best are deeply concerned, and that should raise alarms for everyone.

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