For the past six years, Gordon Propp, who builds sets for British Columbia’s film industry, has kept a close watch over 13 peregrine falcon nests in and around Vancouver, including ten on city bridges. A self-described wildlife enthusiast and citizen scientist, Propp has had a lifelong fascination with these raptors. “To see a creature that high up the food chain adapting to an urban environment, to me, that’s quite remarkable,” says Propp.

Watching peregrines (Falco peregrinus) flit about and hunt with their trademark speed, swooping in pursuit of prey at up to a staggering 400 kilometers per hour (250 miles per hour), is “etched in my mind,” says Propp.

But for the past couple years, most of Propp’s winged wards have been nowhere to be found. Construction and egg predation by clever ravens can probably explain the disappearance at two locations, but he can’t explain why the other nests are empty. 

Propp’s observations are hardly isolated. Scientists around the world have been recording plummeting peregrine populations in at least 11 countries. 

Name any place in the world and peregrine falcons have likely soared across its skies. They breed throughout the eastern United States and northern Canada, as well as in Greenland, Russia, and Scandinavia. They are widespread year-round along North America’s west coast, in South and Central America, Europe, Africa, the Middle East, across Asia, and in Australia. 

In North America, Skip Ambrose, a peregrine expert formerly with the U.S. Fish and Wildlife Service, has been monitoring the falcons along Alaska’s Yukon River since 1973. He, too, has seen a sharp decline in peregrine numbers. In the summer of 2023, Ambrose reported that 20 of 60 peregrine nesting sites were empty, with nearly a dozen more missing a parent. That’s particularly notable because peregrines are generally loyal to both their nesting site and their partner. 

Ambrose’s dire observations kicked Bud Anderson into action. In May 2024, Anderson, a retired peregrine monitor who formerly ran the now-disbanded Falcon Research Group in Washington State, helped launch a forum focusing on the mysterious declines. Since then, more than 100 researchers have joined to discuss hypotheses and share their own observations of dwindling peregrine populations in Denmark, southwest France, Germany, Malaysia, the Netherlands, central Norway, northern Russia, southern Sweden, and Switzerland.

While none of the scientists can definitively say what is going on, Ambrose says nothing has ever killed adult peregrines so quickly—not even DDT, the heavily used pesticide that nearly drove the birds to extinction in the 1970s.

Curiously, the peregrine’s plight in North America seems most pronounced along the coasts. In New Jersey, for example, 22 of the 44 known nesting peregrines went missing last breeding season. In Virginia, local scientists recently noted that a dozen out of roughly 70 birds had vanished. Peregrine nests in inland Washington State, near the Cascade Mountains, seem stable, Anderson says, while those on the nearby San Juan Islands are struggling. 

“It is interesting that coastal populations are showing impact while those in the middle of the continent, so far, do not,” says Patrick Redig, a veterinarian and president of the Midwest Peregrine Society who helps track 200 nesting pairs across seven states.

Though scientists lack an official answer as to what’s driving such sudden and far-reaching disappearances, many—including David Bird, who formerly led the Avian Science and Conservation Centre at Quebec’s McGill University—think highly pathogenic avian influenza (HPAI) may be largely to blame.

Ever since 2022, the virus known as A(H5N1) clade 2.3.4.4b has been spreading around the world, infecting birds, but also leaping to cattle, foxes, seals, and even people.

Bird suspects peregrine falcons could be picking up HPAI after preying on shorebirds, seabirds, and waterfowl—transitory populations that may have been infected on poultry farms.

That HPAI is to blame fits with the observations of Eve Bélisle, who has been monitoring peregrine falcons in Montreal, Quebec, since 2008. Montreal’s roughly 30 peregrines prey on a mix of pigeons, starlings, and other urban birds, but will also go after the occasional waterfowl and shorebird. Necropsies confirmed that at least two falcons in the city died of HPAI last year, while others disappeared, laid infertile eggs, or lost chicks during the breeding season.

Jérôme Lemaître, an avian biologist with the Quebec government, has been tracking the nesting success of peregrine falcons in the province. He says that while peregrines have not been missing from their nests as is the case elsewhere, in 2022 the bird’s reproductive success in southern Quebec did fall from 50 percent to 30 percent, though reproduction rates rebounded in 2023. Lemaître says it’s unclear what role avian influenza may have played in the decline.

Without a large-scale surveillance effort across North America, determining whether avian influenza is driving declines in peregrine falcons along the coast—and in some places even farther inland—is difficult. But Kathy Clark, who leads New Jersey’s Endangered and Nongame Species Program, says that to get a better view of the situation, New Jersey and Virginia state officials may begin collecting and testing the blood of deceased peregrines for HPAI starting this breeding season.

Longer term, Guy Fitzgérald, a veterinarian who launched Quebec’s raptor rehabilitation program, says the province’s peregrine population has plateaued and remains susceptible to further declines until the ongoing outbreak burns itself out.

If HPAI is ultimately driving the declines, Bryan Watts, an ecologist who leads The Center for Conservation Biology at William & Mary university in Williamsburg, Virginia, says North America’s peregrine falcons have a difficult journey ahead. “This disease is just going to have to work its way through, and they’re going to have to develop an immunity.”