Hundreds of dead birds are now being reported as the lethal H5N1 strain of bird flu begins to spread across Australia. Government reports show the virus is circulating in resident birds.
Author
- Bethany J Hoye
Lecturer in Animal Ecology, University of Wollongong
It was a long time coming. The current outbreak emerged in 2020 . By late 2021, it was spreading widely , leaving a widespread trail of devastation among wildlife species across Europe and the Americas. Even as Antarctic birds and mammals began suffering over 2024 and 2025, Australia - and New Zealand - remained free of this viral strain. Until now.
It's easy to picture bird flu as an unstoppable wave set to decimate Australian birds and mammals. But that's not set in stone. Overseas evidence shows the first wave tends to be by far the worst. And deaths from the first wave can vary greatly, with many populations and individuals surviving.
That's good news for some - but it won't save vulnerable species who may lose a large proportion of their global population. For them, recovery is likely to take decades - if at all.
The first wave is devastating
In 1996, a new highly pathogenic strain of bird flu emerged in poultry in China. Periodic outbreaks forced mass poultry culls and sporadic die-offs in wild birds.
In 2020, a new variant - formally known as H5N1 clade 2.3.4.4b - was detected. Soon, it was sweeping through Europe and North America.
It was immediately clear the virus had become more devastating.
Less dangerous strains of bird flu (termed low pathogenic avian influenza viruses) tend to replicate in a bird's gastrointestinal tract, while earlier high pathogenic strains tended to replicate in the respiratory tract.
But this new strain could infect both. Not only that, it was able to infect the central nervous system , leading to neurological disease and death.
The strain has evolved many new variants since 2021. These can now infect over 500 wild bird species, of which over 50% had not had infections previously recorded.
Then came reports of fatal infections in mammals , from scavengers such as foxes to herbivores such as deer and marine mammals such as sea lions. Many had never had bird flu before. The virus was even able to infect livestock such as cattle . Human infections are thankfully rare , with no reports of the virus spreading between people so far.
Across western Europe and North America, deaths of breeding-age adult birds were as high as 25-76% in the worst-hit species. Many populations had near-complete breeding failures .
In 2023, it hit South America. Some bird species lost up to 50% of their global population, while marine mammals suffered their first mass deaths. In Argentina, 17,000 elephant seals died in a single colony .
What happened next?
While mass deaths of wildlife made global headlines, little media coverage has focused on what happened next.
Mass deaths don't seem to strike the same populations year after year, even though the virus is still circulating.
Adult sandwich terns in western Europe died in droves in 2022. But in 2023, mortality rates in adults fell by 96%. That year's chicks were hit hard, but those hatching in 2024 did far better.
Why? Older, more vulnerable birds all died off in the first wave, and surviving terns gained some immune resistance. Younger birds previously shut out from breeding took advantage of the situation, while chicks benefited from reduced competition for food.
Overall, sandwich tern numbers only fell modestly. Even so, the species is projected to take almost 20 years to recover to its pre-outbreak level.
Not all populations or individuals were affected equally - even in the first wave. Atlantic seabirds breeding in large, dense colonies were hit hard . Gannets, terns and skuas all saw mass deaths. But neighbouring petrels, puffins and shearwaters nesting in burrows had far fewer deaths.
This may give some protection to Australia's burrowing little penguins. Authorities are also moving to vaccinate thousands of these penguins.
For many species, bird flu is made worse by other threats. In 2024, Adéle penguins in Antarctica had to battle both bird flu and deadly avian cholera , leading to many losses. Similarly, bird flu hit African penguins already in trouble from the collapse of sardine fisheries.
We can't directly extrapolate from overseas experience. Many Australian birds and mammals are unique. Some, such as black swans, lack crucial immune defences against influenza.
The death toll is not set
Official estimates suggest about 20% of bird and 10% of mammal populations in Australia are at high to extreme risk of population impacts from this strain.
But catastrophic destruction is not guaranteed. Whether a species is hit hard or barely touched will depend on three things: level of exposure to the virus, how well it can resist infection and how well it can cope with being infected.
Before this strain arrived, experts evaluated the risk for all Australian bird and mammal species, developed action plans for important sites and expanded surveillance and diagnostic systems to monitor outbreaks and viral evolution. These measures are globally recognised as the most important tools .
Australia is now following New Zealand in vaccinating captive and closely-managed populations.
Worldwide, no species has suffered 100% losses. That means some individuals likely survived the infection. Generally, animals can cope better with infection if they're well fed, in a good habitat and aren't exposed to pollution or stress.
The biggest lever we can pull is tackling these other pressures, from habitat loss and plastic pollution to invasive species.
We have a chance to try relieving these pressures on wildlife to give them more of a fighting chance - but the window is shrinking.
Bethany J Hoye receives funding from the Australian Research Council.
