Opinion | Why Dead Birds Are Falling From the Sky

Experts are concerned that a new global disease outbreak, possibly worse than Covid-19, might begin any day. The virus that worries them is H5N1, a form of avian influenza, or bird flu. Some researchers have warned that with just a few mutations, or maybe a sudden swapping of gene segments, this deadly flu virus could gain the ability to spread from human to human.

But in fact, the next pandemic has already begun. To use a more accurate term, a panzootic — a widespread outbreak of disease among nonhuman animals — is underway.

To appreciate this catastrophe, we’ve got to move the focus off humans, at least for a bit. H5N1 is devastating the world’s birds. Eagles are dropping dead, as are great horned owls and peregrine falcons and pelicans. Twenty California condors recently died of what’s suspected to be avian flu — 10 are confirmed so far. It’s the worst thing that has happened to wild birds since the pesticide DDT.

Tallying deaths among wildlife in wild places, especially those flying through forests and over oceans, is hard. The Book of Matthew may assert that not one sparrow falls to earth without the knowledge and consent of God, but us mere humans can’t count how many raptors, waterfowl, shorebirds, ravens, parrots and other wild birds may be dying in places beyond notice. The results could be dire for some bird populations, even pushing endangered species closer to extinction. What happens when this killer virus gets into whooping cranes, of which fewer than 900 exist on the planet? What’s next for the California condor, with barely 300 birds alive in the wild?

Seabirds that nest in great colonies are a little more visible. Such dense nesting also makes them more vulnerable to contagious disease, and many kinds are long-lived, maturing at relatively late ages, which leaves their populations slower to rebound. During nesting season last May, at a colony of sandwich terns on the coast of France, observers counted more than 1,000 tern corpses. France as a whole may have lost 10 percent of its breeding population within a week. At remote island sites in Britain, such as Orkney and the Shetlands, the great skua seems to have suffered die-offs of up to 85 percent.

Any such bird flu, so deadly, is called highly pathogenic avian influenza, or H.P.A.I. That label was once applied to viruses that infect chickens. Until this century, these kinds of viruses were virtually unknown among wild birds. The exception was an event in 1961, when 1,300 common terns showed up dead along the coast of South Africa. The cause was a new avian virus of the general sort that — we now know — wild aquatic birds carry endemically and sometimes spill into domestic birds, pigs and humans. For decades after that tern die-off, though, no other influenza so virulent was detected in wild birds. New influenzas did come from wild birds, yes, but in milder form, usually sickening domestic birds little or not at all. Evolving to become more lethal was something that happened, so far as science could see, mainly among farmed poultry.

A Belgian epidemiologist, Marius Gilbert, led a 2018 study of this phenomenon. Dr. Gilbert and his colleagues reviewed 39 cases in which a mild avian influenza had evolved into a killer virus. All but two of those 39 known conversions occurred among commercial poultry.

Is it going too far, I recently asked Dr. Gilbert, to conclude that commercial poultry farms are what deliver the problem of virulent influenzas upon us humans, and also upon wild birds? “No. I don’t think it’s too far,” he said. “But we have to bring nuance to that statement.”

“Commercial poultry” can mean there’s a vast and dense aggregation of birds — thousands, or hundreds of thousands, in industrial-scale operations — or it can mean 10 chickens and six ducks in the backyard of a family in a rural village. The ducks share the rice paddy with wild birds passing through, and some of the chickens are sent live to a local market. Viruses flow in every direction, including to the children who tend the ducks.

The currently circulating H5N1 lineage of avian flu emerged back in 1996, among farmed geese in a rural area of Guangdong province in southern China. Its kill rate among those geese was 40 percent, with symptoms that included bleeding and neurological dysfunction. At some point it passed into wild birds, spreading across Asia to Europe and the Mideast, and occasionally into humans and other mammals, though without triggering long chains of transmission.

In December 2021, it was detected among wild birds in Newfoundland and Labrador, and from there it seems to have been carried by migrating waterfowl down the Atlantic Flyway to the Carolinas, Georgia and Florida. That’s where Nicole Nemeth, a wildlife pathologist at the University of Georgia, encountered it, when bald eagles started arriving dead at her laboratory.

Dr. Nemeth and her colleagues found a high rate of bald eagle nest failure (no surviving chicks) and adult deaths, with dead birds brought to the lab and confirmed to be ravaged by the H.P.A.I. virus. “It was very sad and alarming,” Dr. Nemeth told me.

Adult birds were losing muscle control, shaking their heads, showing signs of weakness or paralysis, keeling over, tumbling from their high nests. Bald eagles are big birds, weighing up to 14 pounds, so when they fall, they land hard. “As a pathologist, I was looking at these birds carefully, and they were clearly dying of a very severe, acute, viral infection,” Dr. Nemeth said. Some were probably dead by the time they hit the ground.

Necropsies revealed organ failure and brain inflammation, but also blunt trauma and bleeding from the long falls. And when the adults became sick and fell, the unfledged young usually died, too, either from the same infection or from orphanhood. In coastal Georgia, during the 2022 season, nesting success for bald eagles was down by 30 percent.

It could get worse. There’s very little, Dr. Nemeth told me, that either science or wildlife management can do. When the bald eagle population declined badly in the 1950s and then the species was declared endangered in 1967, the main cause of reproductive failure — DDT — was banned. The eagle population bounced back, a wondrous conservation success. But you can’t ban a virus like you can a chemical — not a virus that travels everywhere in wild birds and evolves continuously in domestic ones, which are raised in vast numbers on both industrial-scale farms and in backyards.

Our little world contains eight billion humans. It now also contains more than 33 billion chickens. This vast horde of domestic poultry is an important link in the chain of cause and effect that is killing wild birds all over the world. We should consider what can be done about that — if not by wildlife management, maybe by better managing ourselves.

We should think about how our access to cheap supermarket chicken, breasts and legs wrapped in plastic, kindles jeopardy for the hawks, falcons and owls of our forests, the ducks and loons of our wetlands, the vultures that clean away carrion, the crows that amuse us in town, the gulls and terns of our shorelines, the wild geese we enjoy hearing as they honk southward on an autumn night — and jeopardy also for ourselves. We should bear in mind that those 33 billion broilers and roasters represent a great petri dish for the continuing evolution of flu viruses. One such virus could well — just by chance — acquire mutations that make it the next human nightmare. The eagles would still be falling dead off their perches, a tragedy in itself. And the chickens, for us, would come home to roost.

David Quammen is a science writer and the author of “Breathless: The Scientific Race to Defeat a Deadly Virus.”

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