Bird flu has caused catastrophic die-offs – could a wave of extinctions follow?

It began with the brown skuas. The migratory seabirds had arrived on the sub-Antarctic island of South Georgia in 2023, harbouring a deadly passenger. Within months, the southern elephant seals that had come ashore to breed became sick. Mothers and newborn pups were weak and trembling, with many struggling to breathe. What followed was a horrific mass casualty event, caused by the highly infectious avian influenza (H5N1).By 2025, more than 53,000 female elephant seals had died on South Georgia. That is especially alarming, scientists say, because the island is home to more than half of the global breeding population.A highly potent strain of the bird flu virus, which first began circulating in the northern hemisphere in 2021, has now spread to four of the five main southern elephant seal populations. In some South Atlantic colonies, more than 90% of newborn pups have died, alongside about half of all adult females.A southern elephant seal with a pup at Gold harbour, South Georgia. By 2025, more than 53,000 female elephant seals had died on the island. Photograph: Ashley Cooper/Alamy“This is a horror show for wildlife,” says Ruth Cromie, coordinator of a UN taskforce on avian influenza and wildlife. “The scale of mortality is absolutely unprecedented.”In April, the mass deaths prompted the IUCN red list, the global authority responsible for assessing the extinction risks of wildlife, to downgrade the southern elephant seal’s conservation status by two categories, from “least concern” to “vulnerable”.It was the first time a species has had its extinction risk raised explicitly because of H5N1.Dead elephant seals on South Georgia in January 2026. Photograph: Jane YoungerThe IUCN listing notes that scientists suspect the global southern elephant seal population has declined by nearly a third, and models suggest many populations will take decades to recover. So far, scientists have found few antibodies to the disease among surviving animals.The southern elephant seal is just one species that has suffered catastrophic die-offs since H5N1 began burning its way through the world’s wildlife, killing hundreds of thousands of animals, if not millions. Avian influenza is now a threat to at least 40 bird species, according to BirdLife International, a global conservation partnership. And yet, no other animal – seal or bird – has seen its conservation status changed.But scientists warn that does not mean there are not more species being pushed to the brink by this deadly pathogen.Assessing a species’ conservation threats is a painstaking scientific and bureaucratic process. To date, the red list has analysed nearly 176,000 species’ extinction risks, and aims to reach 260,000 species by 2030, as well as 142,000 reassessments.Most species are only reassessed by a panel of expert scientists once every decade or so – no matter the emerging threats they may face. The red list itself is updated two to three times a year.Birds are the exception. BirdLife International serves as the authority for assessing the world’s 11,000-plus bird species, reviewing hundreds of birds every year. For that reason, our understanding of the extinction risks of birds is better than for any other animal group. In an epidemic where birds are the foremost victim, that is obviously convenient.But there can still be a lag when deaths are coming faster than scientists can convene and publish under peer review. “We still need to get these things out there,” says Rob Martin, senior red list officer at BirdLife International.Northern gannets rest next to dead gannets on Rouzic Island, off the Brittany coast, in 2022. Photograph: F Tanneau/AFP/GettySpecies are generally “uplisted” when they have experienced a sustained population decline over three generations, a small population is in decline or living within a restricted range, or calculations show that the species could become extinct in the near future, given existing population dynamics.Scientific uncertainty often underlies such reviews. Assessors need to know the animal’s baseline population before they can determine if it has crossed a threshold towards extinction. For many species, such figures are in short supply. And without knowing a species’ starting point, it is difficult to calculate the true toll of an outbreak.Scientists with BirdLife International say they are now focused on reassessing four species that have suffered H5N1 outbreaks large enough to affect their conservation outlook: the great skua, northern gannet, Peruvian pelican and Humboldt penguin.Great skuas courting on the Shetland Islands, off Scotland. Photograph: BSIP/Universal/GettyIn Scotland, home to more than half of the world’s great skuas, the population of monitored colonies fell by 76% between 2021 and 2023.Meanwhile, the northern gannet, the largest seabird native to the North Atlantic, declined dramatically in two breeding populations in the UK as bird flu spread: by 43% at Bass Rock in Scotland between 2021 and 2024 and 47% at Grassholm in Wales between 2015 and 2024.These are two of the largest northern gannet colonies, and extrapolating from such figures would imply a steep population fall, say BirdLife International assessors, potentially qualifying the species for uplisting from “least concern”.But when scientists took a broader look, they found that the entire north-eastern Atlantic population had declined no more than 17% between 2013 and 2024. “Rushing to judgment a few years ago, with only partial data available and no sense of whether the whole population was declining, would have been premature and could have led to a misclassification,” BirdLife International scientists say.Martin cautions that it is important to prevent “kneejerk” uplisting after a mass mortality event, as listings such as “critically endangered” can trigger dozens of conservation funding proposals and divert government spending. “We need it to be a tool that the community can reliably use and say ‘we trust this,’” he says.Municipal workers collect dead pelicans in Lima, Peru, in November 2022 as another struggles to move. At least 13,000 birds had died by then along Peru’s Pacific coast. Photograph: G Pardo/APAbout a fifth of the pelican population living in Peru’s marine protected areas are thought to have succumbed to bird flu in 2022 and 2023. But the Peruvian pelican is considered an abundant bird.“It’s difficult to get accurate numbers for the population pre- and post- [the H5N1 outbreak] because they’re in quite isolated areas without sufficient funding for monitoring,” says Martin.Counting the dead often poses an even greater hurdle, especially for migratory species that travel vast distances over oceans. “You usually don’t find most of the dead birds. If they die at sea, they’re lost,” says Wouter Langhout, a policy consultant at BirdLife International.The flightless Humboldt penguin, which spends most of its life on land, has made such tallies easier. In 2023, scientists counted nearly 3,000 dead penguins along Chile’s coasts, prompting another review of the already vulnerable species, which was last assessed in 2020.Even when population and mortality numbers are easy to come by, spectacular mass die-offs – as gruesome as they may be – do not guarantee a species is inching towards extinction. For long-lived seabirds, large waterbirds and elephant seals that are slow to reproduce and late to sexually mature, a large loss of breeding adults can translate into severe population declines in just three generations – a key red-list criterion for uplisting.But fast-reproducing animals or those with abundant populations may be spared such long-term consequences, especially if more adult males die than fertile females.Antarctic fur seals in heavy snow on South Georgia as king penguins pass by. Photograph: Ben Cranke/naturepl.comIn the case of the Antarctic fur seal, for example, far more males have succumbed to the disease than breeding females, says Jaume Forcada, a marine mammal expert and population biologist at the British Antarctic Survey who has studied Antarctic fur seals for 25 years.That is because male and female fur seals behave very differently. Whereas the males come ashore for about two months, often battling with one another on beaches, “when females arrive, they pup, and then they go out very quickly. They’re less exposed to other animals apart from the moment when they are rearing their pup,” Forcada says.In turn, “the males were very severely affected, whereas the females were absolutely fine. It’s like it was a completely different species. It hadn’t affected them.”If scientists uncover a population decline that would warrant uplisting, attributing it to avian influenza may not be cut and dried, Martin cautions. For example, BirdLife recently reassessed the African penguin, which was heavily affected by avian influenza.Although it was uplisted to critically endangered in 2024, assessors determined that food loss linked to commercial fisheries and the climate crisis – not bird flu – ultimately drove its decline.The carcass of an endangered Humboldt penguin on a beach in Vina del Mar, Chile, in 2023. Photograph: R Garrido/ReutersThe 3,000 dead Humboldt penguins in Chile have also triggered debate among scientists. Penguins often drown in gill nets and 2023 was also an El Niño year, which may have had a hand in the birds’ deaths.“That year could be described as a ‘perfect storm’, which produced highly unfavourable conditions for Humboldt penguins,” says Alejandro Simeone, a scientist in the IUCN’s penguin specialist group.“While it is quite possible that some Humboldt penguins died from [avian influenza], current evidence makes it difficult to assess the true extent of the virus’s role in this decline,” Simeone says.In the end, it may not be the most visible victims that are in the greatest peril. Birds with tiny isolated populations – often those inhabiting small islands – can quickly move towards extinction even if only a handful of individuals die.“For species that are already under threat and declining for other reasons, and then you put in something like avian influenza, that might be the thing that then pushes them over the edge,” Cromie says.To date, scientists say they are not yet aware of any such cases from bird flu, but are anxiously monitoring Australia, where H5N1 was first detected on the mainland in June.The endangered regent honeyeater in an aviary at Sydney’s Taronga zoo this year, where vulnerable species of birds are being vaccinated against bird flu. Photograph: B de Marchi/AAP“A lot of Australian species still do have quite wide ranges, but there’s a very specific habitat that they rely on,” Martin says.The country is also home to a number of critically endangered birds that can be found nowhere else in the world, such as the orange-bellied parrot and the regent honeyeater.Waiting for uplisting, Cromie adds, may miss the bigger picture. “We have to be focusing on taking these other pressures off,” she says. “If we become very fixated on only waiting to do something because of a change of status, there’s such a lag there. We’ve got to be doing other things at the same time.”Already, fur seals and dolphins in Australian waters have begun to die from bird flu. And things are about to get worse for the now-vulnerable southern elephant seal.In August, the government’s Australian Antarctic Division said it would evacuate its research station on Macquarie Island in anticipation of mass seal die-offs if H5N1 reaches its shores. Macquarie is the last stronghold of southern elephant seals still untouched by the virus.Though the risk to human health is low, the agency said it hoped to prevent psychological distress for staff who could witness the deaths of thousands of newborn seal pups.About 5,000 of the world’s largest little penguin colony, on Australia’s Phillip Island, were vaccinated against H5N1. Photograph: GettyThe next update to the list is due on 26 November, and proposals for new uplistings will next be discussed in January and February 2027.“Our biggest fear is always that we completely miss something that suddenly is critically endangered. That’s what we’re guarding against,” Martin says.Find more age of extinction coverage here, and follow the biodiversity reporters Rebecca Ratcliffe, Phoebe Weston and Patrick Greenfield in the Guardian app for more nature coverage.