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Story Publication logo September 27, 2023

Flight Risks: Changing Bird Migrations Threaten To Bring New Infectious Diseases to Humans

A castor bean tick (ixodes ricinus) on grass, possibly carrying disease

For years researchers have predicted that human health will suffer as the climate warms...

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Migratory birds efficiently ferry pathogens around the world. As a warming climate reshapes their journeys, infectious disease experts are on guard for new threats to humans

GERMANY, SWEDEN, AND THE NETHERLANDS—One day in May, a team of scientists parked their cars in a German nature park near the Baltic Sea and stared at the top of a 30-meter-tall black poplar tree. They were seeking eagles—and clues to the stream of viruses and other pathogens coursing along Europe’s flyways in the guts of migratory birds. These white-tailed eagles don’t migrate much. But the adults capture and eat birds that do, and feed the prey to their offspring.

It was a month or so into the breeding season, and the scientists hoped to find nestlings—far easier to trap than adult eagles. A local eagle watcher pointed to a nest at the top of the tree. The signs were good. In the distance, a white-tailed eagle soared in the clear sky, and Anne Günther, the veterinarian leading the project, smiled as she pointed to plaques of white goop staining the grass: fresh eagle feces. It was time to take a closer look at the nest.

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Veterinarian Oliver Krone, who is based at the Leibniz Institute for Zoo and Wildlife Research and has studied white-tailed eagles for 23 years, deftly hovered a drone over the nest and saw three eaglets. A tree climber dressed in camouflage slingshotted a rope over a sturdy branch and scurried up it, carrying a sack. Soon the fluffy gray eaglets and the dead prey in the nest had been lowered to the ground. The researchers banded, swabbed, and drew blood from the eaglets, all to be analyzed for pathogens they might have picked up from their migratory prey and for antibodies from their mother, signs of past infections. They also sampled the carcass of a wigeon, a dabbling duck, that was found in the nest.

Ducks and other migratory birds efficiently transport all kinds of microbes around the world, including novel influenza strains that threaten poultry and humans, West Nile virus, and a wide range of bacteria that can sicken people and harbor antimicrobial resistance genes (see sidebar). Günther’s project, which has already detected a devastating bird flu strain in the eagles, is funded by the Versatile Emerging infectious disease Observatory (VEO). A $30 million Europe-wide collaboration, VEO aims to improve the early warning system that tracks key pathogens, potentially thwarting pandemics. “We’re trying to figure out what do hot spots look like for human risk,” says VEO project coordinator Marion Koopmans of the Erasmus Medical Center.

A staggering number of variables influence the risks: bird behavior, the nature of the pathogens, the insects and other vectors that help spread them, and humans’ own habits and effects on the landscape. And as the world warms there’s a new variable to consider, says Martin Beer, a veterinarian at the Friedrich Loeffler Institute (FLI) who is Günther’s Ph.D. supervisor. “Bird migration, breeding, and everything is connected to climate change.”

As birds migrate to feed or breed, rising temperatures and changing moisture patterns are likely to affect where they go, how long they stay, and what pathogens they meet. The VEO group is on particularly high alert for birds that travel through Europe to the Arctic, which is warming faster than any other part of Earth and serves as a mixing pot for many species.

Already climate change is shaping some birds’ journeys. What that implies for the airborne traffic of pathogens, and ultimately the risk to people, remains uncertain, however. “It’s not as simple as when this happens, that will happen,” Koopmans says. But untangling the relationships—and learning what warning signs to watch for—is critical, she adds. “Can we narrow down the multitude of factors that do drive disease emergence to a set of indicators that you can build into a monitoring system?” Koopmans asks. “I’ve spent decades now chasing outbreaks. It’s not good enough. Once it starts spreading, it’s too late.”

THE FIRST STEP is to learn where specific birds go and what they’re carrying. Each morning this spring, a dozen college-age students who live at the Ottenby Bird Observatory in Sweden repeatedly walked around the nature preserve, clapping their hands in unison. What from a distance resembles a New Age ritual serves a most practical function: It compels any nearby birds—in this case small migrants called wheatears—to fly down large funnels so they can be captured for measurements and banding.

The observatory sits at the tip of Öland, an island in the Baltic Sea about 300 kilometers as the wigeon flies from the park in Germany where the white-tailed eagles reside. Each spring and summer, birds arrive in Ottenby’s 340 hectares of lush coastal grassland. For some, it’s a final destination to breed, but for many others it’s a stopover on their way north to breeding grounds in Iceland or the White Sea in Russia. On their way south in the fall, the birds often stop in Ottenby again, before continuing as far as Africa.

During both seasons the birds foul water bodies with the viruses in their guts, infecting other birds that then travel on different flyways. The result is a global relay system for transmission. “You can easily imagine that [wild birds] have the potential to bring new pathogens to new areas,” says ornithologist and microbiologist Jonas Waldenström, who works about a 1-hour drive away at Linnaeus University (LNU) and sometimes collaborates with VEO researchers.

An eaglet was lowered from its nest in Germany to be banded and tested for pathogens from the migratory birds it dines on. Image by Jon Cohen.

For over 2 decades, Waldenström has sampled more than 100 species of birds at Ottenby. He has found a gallery of pathogens that threaten poultry, humans, or both: influenza and Newcastle disease virus, CampylobacterSalmonellaYersinia, tick-borne encephalitis virus, Candidatus, BabesiaRickettsia, and papillomavirus.

Waldenström and his colleagues have also spotted signs that a warming climate is beginning to affect the birds’ travels. “We are quite poor at remembering how things were 20, 30 years ago, unless we have some notes,” he says, as barnacle geese bark in flight above him. But this observatory has trapped and recorded birds—140 species at current count—for 77 years.

The staff band the animals, trusting that the enormous international community of birders will report where they are spotted next. Birders are obsessive observers. Once you become fascinated with birds, Waldenström says, “in a way you will never be alone again, because there are all these little fluffy dinosaurs around at all times. When people get this disease, there is no real cure for it.” Many share what they find on websites, and the crowdsourced data have traced annual flyways. They are also revealing changes.

2006 Science paper Waldenström co-authored analyzed data from 1980 to 2004 to show long-distance passerine birds were arriving earlier in the spring at Ottenby and four other European observatories, likely as a result of climate change. During that period, the spring “green up” came about half a day earlier each year on average, and April and May were typically warmer, likely leading to more of the insects that many of the birds eat. “They’re good heralds of what is going on,” Waldenström says. “We all see that climate change is happening. But long before we started to see and feel these changes, you can see those types of changes in the bird community.”

Waldenström’s team now equips black-headed gulls and mallards with transmitters that cellphone towers and satellites can track. “Bird banding is a rather blunt tool,” he says. “Typically, you only get few recoveries, and in most cases it is just the site of banding and the site of recovery.” The transmitters should give a fuller picture of migration routes and timing—but they cost about $1000 each and are cumbersome, which limits the number and types of birds they can follow. Smaller birds cannot wear the tags at all.

JUST HOW CLIMATE-DRIVEN changes in bird migration affect human, or even avian, disease risk depends on an intricate web of factors. Many long-distance migrants start their journeys in response to signals unaffected by climate, such as changes in daylight or internal cues. Keeping to their traditional timing, they might reach a breeding area when spring is more advanced, encountering a changed ecosystem and, perhaps, different pathogens. Others, responding to temperature and humidity, might speed up their migrations, expending more energy, which might also make them more apt to pick up diseases. “Migration is energetically expensive, and studies show that this suppresses the immune system,” says Mariëlle van Toor, a movement ecologist who works with Waldenström at LNU.

For some birds, climate change might convert what once was a stopover spot into their primary breeding grounds, which could make them more likely to pick up certain pathogens. An evolutionary theory known as the dilution effect holds that a diversity of species, expected at a stopover point, can reduce the ability of a pathogen to spread, because an infected bird is more likely to transmit it to a different species in which it does not thrive. But if large numbers of one species settle in to breed, this safeguard may diminish.

Crowded skies

Many migratory species stop in the coastal meadows around Sweden’s Ottenby Bird Observatory on their annual journeys between Arctic breeding grounds and winter habitat. Banding and tags have traced some species’ routes and revealed early signs of change.