25,669 Northern Gannets in Canada.
134 harbor and gray seals along the Maine coastline.
21 California Condors in the western United States.

These figures represent just a small portion of the wildlife casualties from a highly pathogenic avian influenza strain—commonly known as bird flu. The H5N1 virus has recently surged worldwide, catching scientists off guard with its rapid and unpredictable spread. While the public may worry about the rising cost of eggs or the threat of a human pandemic, a staggering number of wildlife deaths are occurring largely unseen, with the numbers so vast they are often hard to comprehend.

“It’s tempting to just see these numbers as abstract and not fully grasp what they signify,” remarks Stephanie Avery-Gomm, a conservation scientist at Environment and Climate Change Canada. “However, when you really pause to think about it, it’s quite heartbreaking.”

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Avery-Gomm experienced the grim reality firsthand in early 2022 when the lethal bird flu strain hit North America. Northern Gannets, which traverse the Atlantic and spend most of the year at sea, began appearing dead on shores, especially during their spring breeding season at six eastern Canadian colonies. Unable to pinpoint the source, scientists deployed a helicopter to survey the largest breeding colony, producing footage that still evokes strong emotions from Avery-Gomm.

“The footage revealed a catastrophic loss,” she explains. “So many dead gannets everywhere.”

From collected reports of fatalities, Avery-Gomm and her team estimated that the virus killed 25,669 of Canada’s 213,704 counted breeding Northern Gannets in just six months—decimating the population. Overall, more than 40,000 wild birds perished in the regional outbreak, with Common Murres being the second most affected species, tallying over 8,000 deaths. “Nothing could have prepared us for this level of mass mortality,” admits Avery-Gomm.

The subsequent data were even more disheartening. Wildlife scientists have always known they were only seeing a fraction of the actual bird flu casualties, especially at sea, which is harder to monitor. Since the outbreak began, significantly fewer birds have been observed at the breeding colonies compared to previous years. At the largest Common Murre breeding site in the area, numbers have dropped by 9 percent since before the outbreak; Northern Gannet numbers have plummeted by about 40 percent across Canada. “We now have far fewer gannets in North America than we did back in 2021,” Avery-Gomm states somberly.

A Completely Novel Form of Bird Flu

7,000 Snow Geese in Idaho.
2,712 Humboldt Penguins in Chile.
9,600 Sandwich Terns in the Netherlands.

Bird flu viruses have been circulating for centuries, appearing in historical records as “fowl plagues,” notes Wendy Puryear, a scientist at Tufts University who monitors influenza viruses in wildlife. Waterfowl like ducks and geese often serve as reservoirs for the virus, though domestic poultry are highly susceptible as well.

In crowded farming conditions, poultry are extremely vulnerable. Lethal strains of avian flu can decimate 75 percent or more of a flock within days, qualifying them as high-pathogenicity avian influenzas—a label traditionally reserved for mortality in farm birds, not wild ones.

Puryear has been tracking avian flu strains in wild birds for decades, vigilant for potential spillovers into poultry and humans. Historically, how wild birds cope with the virus has been of minimal concern—wild birds and waterbirds have hosted flu strains for ages without significant issues. “There’s a vast array of influenza viruses circulating in nature among wild birds, and most don’t seem to cause much disease,” Puryear explains. “We haven’t observed mass die-offs or impacts on their migration patterns—nothing that has been detectable.”

“We are entering unknown territory.” —Wendy Puryear, scientist, Tufts University

However, influenza viruses are notoriously unpredictable. Their genetic material, composed of eight segments of RNA, can easily reshuffle into new combinations when two different flu viruses infect the same animal. This mixing can produce novel strains that cause more severe sickness, spread more readily, or adapt better to specific species.

Scientists trace the lineage of the H5N1 virus that devastated Northern Gannets back to a goose in southern China in 1996. Over the past three decades, the virus has traveled globally, swapping genes with local flu viruses along the way. In 2020, as the COVID pandemic ravaged human populations worldwide, a new lineage of bird flu viruses known as 2.3.4.4b emerged and spread across parts of Africa, Asia, and Europe. By late 2021, this deadly strain had crossed the Atlantic, first appearing in Canada before reaching the U.S.

This variant represents an entirely new challenge. “We are navigating uncharted waters,” Puryear asserts. “It’s behaving in ways we’ve never seen with flu before, and there are no indications it’s going away anytime soon.”

Within months of its arrival in North America, the virus was detected in wild mammals—both terrestrial and marine—in both the U.S. and Canada, as more wild birds were found dead. The virus then rapidly spread to South America and even reached Antarctic islands and the mainland by early 2024. The severe Antarctic winter did not eradicate the disease, which resurged during the subsequent southern summer. “The entire Antarctic peninsula is now experiencing outbreaks,” reports Marcela Uhart, a wildlife veterinarian at the University of California, Davis.

To date, at least 406 wild bird species and 51 wild mammals worldwide have contracted the virus. Australia remains the only continent untouched by the disease.

As the virus has spread, it has devastated some species and regions while sparing others. “The situation is incredibly complex,” observes Brian Millsap, a raptor ecologist at New Mexico State University. “It flares up unexpectedly in one place, then fades… only to reappear elsewhere.”

The Unseen Declines

17,400 southern elephant seal pups in Argentina.
2,286 Dalmatian Pelicans in Greece.
24,463 Cape Cormorants in South Africa.

While scientists have witnessed glimpses of the virus’s destructive impact, the general public remains largely unaware of the significant mortality occurring often in remote parts of the world.

“This is a massive event, yet it’s virtually invisible,” states Uhart. She observed firsthand as avian influenza ravaged a large breeding colony of southern elephant seals in Argentina in late 2023. Among the year’s pups, 96 percent perished—approximately 17,400 animals. Even a few adults succumbed at the colony, which is an atypical occurrence.

And like Avery-Gomm’s team, Uhart and her colleagues have only received increasingly dire news in the aftermath of the outbreak. By late 2024, only one-third as many adult females arrived at the colony’s most densely populated beaches to breed compared to previous years. “Instead of seeing long lines of animals and hearing their calls,” says Claudio Campagna, a wildlife conservationist, “the beach was silent with only a few animals present.”

The significant reduction in numbers at the colony suggests that many adult elephant seals likely died of avian influenza at sea, out of scientists’ view, Campagna notes, who collaborated with Uhart to model potential recovery scenarios. “It could take a century to return to the numbers we saw in 2022,” he predicts.

Fortunately, many of the mammals reported ill or dead from avian influenza in the U.S. are of common species. Infected red foxes, coyotes, and raccoons, for instance, have been frequently observed—but not nearly at the scale of the marine mammal mass mortalities. These are abundant species, points out David Drake, an urban wildlife ecologist at the University of Wisconsin-Madison, so he isn’t overly concerned.

Other species are less lucky. Bald Eagles were among the early victims of bird flu, and isolated populations continue to suffer. Between January and June of 2022, 136 dead eagles were confirmed to have avian influenza across 24 states. Rebecca Poulson, a wildlife disease researcher at the University of Georgia, monitored the outbreak along the Georgia coast. “The reports of devastation from the field were truly sobering and daunting,” she recalls.

Bird flu has also impacted Bald Eagle populations in the Great Lakes region, where Bill Bowerman, a wildlife ecologist and toxicologist at the University of Maryland, has been studying the birds for 40 years. Here, too, devastation prevails. In Minnesota’s Voyageurs National Park—often called “eagle nirvana”—the iconic bird has become much rarer than just a few years ago. Across the park, researchers found only four chicks last year.

Breeding adults are scarce as well. “Two-thirds of the nesting pairs are gone,” Bowerman reports. “It may take three decades for them to recover.”

The Limits of Data

5,500 Peruvian Pelicans.
600 Arctic Tern chicks in the U.K.
At least one walrus, possibly six, on the Norwegian Svalbard islands in the Arctic.

The counts of known fatalities and the estimates of missing breeders at colonies are heartbreaking, but there is a third, even more distressing number: the count of unseen deaths. “A lot of mortality happens in wildlife and no one sees it,” Poulson explains. “These events could be occurring in vast areas of the country where there simply aren’t enough human observers to detect and characterize them.”

Much of what Bowerman knows about Bald Eagles in the Great Lakes, for example, comes from the national park and from five sites he monitors as part of a pollution remediation project, using eagles as indicators of ecosystem health. The rest of the nation lacks such comprehensive monitoring.

And the problem isn’t confined to Bald Eagles, Millsap adds. Quite the opposite: Bald Eagles and Peregrine Falcons actually have more monitoring than most species he studies, following their near extinction in the 20th century. Other species—Merlins, Cooper’s Hawks, Sharp-shinned Hawks—may be equally vulnerable to bird flu but have never come as close to extinction. This means they lack any dedicated surveys at all, so there’s no sense of local or even regional declines. “The bottom line is, we’re in a situation where we don’t know, and we may not have a mechanism to really find out unless it’s massively catastrophic,” Millsap says.

To truly understand the effects of avian influenza, these monitoring programs needed to be in place before the outbreak began, says Frank Baldwin, a waterfowl biologist at the Canadian Wildlife Service who studies Snow Geese. Because hunters target these birds, the government tracks them with a program that involves placing ID bands on individuals at their nesting sites across the Arctic. When a hunter kills a banded Snow Goose, they report it to the government, providing scientists a glimpse of that animal’s story.

The strategy has limitations: in the first few seasons of avian influenza, the team wasn’t able to band many birds due to the COVID pandemic, but the hunting data appeared normal. Then this spring, Baldwin began receiving more reports of dead geese, but he won’t have any data until hunting season starts in the fall.

Still, having some data is better than none. And such programs need to be established long before any unusual event begins to offer useful insights. “You can’t just create these monitoring programs in a few years; their value lies in their long-term nature,” Baldwin emphasizes.

Ecosystems in Flux

A polar bear in the North Slope of Alaska.
1,621 Caspian Terns in Washington State.
3,500 northern fur seals in southeast Russia.

Understanding the numerical impact on individual populations and species is just the first step in grasping the outbreak’s scale.

Uhart is concerned that the devastated southern elephant seal colony in Argentina won’t be able to breed as successfully in the future due to the high number of deaths, which could have broader implications. She has already observed changes to the intricate harem system that governs breeding at the colony. “The entire reproductive system was disrupted,” she notes. “There was no social structure left.”

Large-scale animal die-offs could also destabilize whole ecosystems, as deaths occur within a network where each species occupies specific niches. The Northern Gannets of Canada, for instance, act as apex predators in the ocean, according to Avery-Gomm, preying on fish such as mackerel and herring. With fewer gannets to consume them, fish populations may increase, potentially disrupting local ecological balances.

“There are so many stressors already; this is just another one they really didn’t need.” —Johanna Harvey, wildlife disease ecologist, University of Rhode Island

During breeding season, the birds are on land, depositing nutrients they’ve consumed from the ocean into terrestrial ecosystems through their droppings. Each altered dynamic can send ripples deeper into the ecosystem, often in ways too subtle for scientists to detect.

And then there are the carcasses. Every ecosystem has mechanisms to break down dead animals, but death on the scale of avian influenza can overwhelm those systems. Indeed, scavengers such as Black Vultures and raccoons in the U.S. have been particularly affected by the virus, likely from attempting to consume infected corpses. “Anything that might be feeding on infected individuals or carcasses, those are kind of sentinel species that give you an indication of how much virus is actually present in the environment that you’re not detecting,” explains Johanna Harvey, a wildlife disease ecologist at the University of Rhode Island.

A different ecological crisis illustrates the potential costs of losing scavengers. In India, a veterinary treatment used in cows decimated local vulture populations that fed on bovine carcasses. In much of the country, vultures nearly vanished—and now scientists estimate that human death rates increased by four percent in these areas from diseases spread by carcasses that vultures were no longer scavenging. Uhart worries that similar issues could arise due to bird flu—particularly in Antarctica, where Brown Skuas that eat seal carcasses, penguin chicks, and seabird eggs have been severely affected. Especially in Antarctica’s harsh climate, “if there are no scavengers to remove dead carcasses, well, the virus might just persist there,” she says. “All those carcasses will be everywhere.”

Fears of Extinctions, Glimmers of Hope

It is not yet clear whether avian influenza will drive any species to extinction, but it is a close possibility for some. Overall, one in six of the bird species and a quarter of the mammals affected by avian influenza are considered near threatened or worse by conservationists, according to research from Sergio Lambertucci, an ecologist at Argentina’s national science agency, CONICET, and at the National University of Comahue.

Lambertucci also highlights a concerning case: a California Condor—one of the rarest birds in the U.S.—was found dead from the virus in March 2023. Officials were so alarmed that they took the unprecedented step of vaccinating condors against bird flu, but the damage had already occurred. Before the vaccines were ready, 21 of the fewer than 600 living birds had died in the outbreak.

Uhart is particularly concerned about the ongoing toll of infections among the 22 species of albatross, which are among the most threatened groups of birds globally—and about what could happen if the virus reaches the approximately 1,600 endangered Hawaiian monk seals still alive.

After all, bird flu is not the only threat these struggling species face. “There are so many species already in decline,” Harvey notes. “There are so many stressors that this is just an additional burden they really didn’t need.”

However, it’s not all doom and gloom. Many researchers have detected protective antibodies to avian influenza in wildlife and have found them in some animals. “There’s good evidence from our group and many others that these animals, if they survive, can mount an immune response,” Poulson says of the eagles and many other species as well. No one knows yet how effective that immunity is or how long it can last, but it’s a promising sign that some infected animals are surviving and may be better prepared to withstand another infection. “And that’s true for many different species,” Poulson adds.

That’s a small ray of hope as animals continue to fall ill. Among the most recently reported infections of bird flu in U.S. wildlife: a round-tailed ground squirrel and a desert cottontail in Arizona. Red foxes in Colorado, New York, and Massachusetts. A pair of Common Eiders in Maine. A Gambel’s Quail and a Black-necked Stilt in Arizona. Five Black Vultures in South Carolina.

All these numbers? They add up.