Waterfowl—ducks, geese, and swans—are the primary host of the viruses, and large animal agriculture facilities are often found in close proximity to their remaining wetland habitats. For instance, California’s Central Valley and the East Coast’s Delmarva Peninsula are both critical wintering grounds for waterfowl, along major North American bird migration routes, and epicenters of U.S. poultry production.

As a result, some scientists who track waterfowl question whether this geographic overlap—alongside the shrinkage of waterfowl habitats—creates more opportunities for the virus to spread between infected waterfowl and the animals in agricultural facilities.

In theory, the ongoing destruction of wetlands could also lead waterfowl to concentrate on the remaining “small postage stamps of habitat,” potentially driving the spread of the virus between birds too, said Michael Casazza, a research biologist who tracks waterfowl at the U.S. Geological Survey’s Western Ecological Research Center.

“The basic idea is that the more you concentrate animals into a small habitat, there’s probably a greater opportunity for transmission between individuals, and then the greater chance for disease spread within waterfowl,” said Casazza.

Casazza and other wildlife scientists have documented waterfowl using agricultural facilities—from foraging for grain in feedlots to roosting in effluent ponds—but questions remain about why the birds seek out the facilities and the potential connections to the spread of avian flu. It is, however, broadly understood that biodiversity and habitat loss tend to drive the spread of infectious disease between animals, which has also been linked to the risk of spillover.

These are important questions considering how devastating the circulating H5N1 strain has been for the U.S. poultry industry, resulting in the death of over 90 million domestic birds since January 2022, when the virus was first detected in wild ducks in the Carolinas and then soon after in poultry. It also has spread to the cattle industry, infecting 42 herds to date. And while the Centers for Disease Control (CDC) still considers H5N1 to be low-risk to humans, the only two cases of the virus infecting humans in the U.S. have been farmworkers in the dairy and poultry industries.

“You really don’t want your poultry industry to be centered in really important wintering waterfowl areas where they’re concentrating in high numbers,” said Jeff Buler, who leads a research laboratory at the University of Delaware that uses weather surveillance and radio tags to track waterfowl. “Unfortunately, they overlap a lot.”

Buler is currently tracking wild geese that winter in the Delmarva, along the thousands of small wetlands known as Delmarva bays. These serve as an “important stopover and breeding ground for millions of waterfowl, waterbirds, and migratory birds,” according to the U.S. Fish and Wildlife Service. The peninsula is also home to the $4.4 billion Delmarva chicken industry, which raised 600 million chickens last year on more than 1,000 farms, according to the Delmarva Chicken Association. Sussex Country, along the peninsula in southern Delaware, is the largest broiler chicken-producing county in the nation.

The poultry farms often grow corn and soy for chicken feed. Buler has been tracking snow geese and Canadian geese using telemetry, affixing a radio transmitter to the geese, which has allowed him to observe the wild birds venturing onto the poultry farms. With this highly precise technology, he has observed that these feed fields often attract waterfowl, who eat the leftover stubble. He’s also observed them using the farm ponds, nearby the long barns that house the poultry.

Over the last four winters, Buler says he has documented that one-third of all poultry facilities on the Delmarva have had radio-tagged Canada geese or snow geese spend time within 2 kilometers of farms. He also found that 84 percent of the 68 geese he is tracking have spent time within 2 kilometers of a poultry farm.

The wild birds don’t need to be directly in the barn to spread the virus. It more often spreads indirectly, such as through contaminated feed, footwear, and equipment. Given this indirect spread, the relationship between the proximity of wild birds to poultry facilities and the risk of avian flu remains unclear, a question Buler recently proposed studying in a grant application to the U.S. Department of Agriculture (USDA).

“We’re seeing higher prevalence of avian influenza in the environment, namely in water and soil associated with these water bodies, in watersheds, where there are a lot more waterfowl,” said Buler. “So, the concern there is that even though the waterfowl might not be in very close proximity—like directly interfacing with the poultry farms—they may be shedding virus into the environment.”

Like the Delmarva, California’s Central Valley is another critical wintering spot for millions of waterfowl. Given that the water typically doesn’t freeze, birds often settle there for months. “The Central Valley is kind of a one-stop shop for wintering birds,” said Casazza. Yet, it’s also a major agricultural basin, formed by draining and diking wetlands, the habitat of waterfowl. This low-lying stretch of land produces a quarter of the U.S. food supply, including as a major supplier of poultry, home to more than 600 commercial poultry farms.

In 2021, Buler and a team of researchers mapped waterfowl distribution relative to commercial poultry farms in the Central Valley. They developed a model that predicted that in the Central Valley, when the waterfowl population peaks in January, the exposure risk to commercial poultry facilities sharply increases—to a point where a third of the poultry farms were highly exposed to waterfowl.

“We have a lot of waterfowl that spend their winter [in the Central Valley], which is not a bad thing, but it just happens to be where we grow all of our commercial poultry,” said Maurice Pitesky, an associate professor at the School of Veterinary Medicine at the University of California, Davis, whose research focuses on disease modeling of the spread of avian flu from waterfowl to domestic poultry.

“Historically, that happened because the Central Valley is flat, the land [was] inexpensive, and it’s easy to transport corn and soy from the Midwest,” said Pitesky. However, “We’ve never really thought, ‘Oh, maybe we shouldn’t be growing commercial poultry right next to habitat for waterfowl.”’

But now, those consequences may be coming home to roost—quite literally—in the form of waterfowl roosting on agricultural facilities.

Wild ducks have been called the “Trojan horses” of H5N1 for their ability to spread the virus, sometimes without showing symptoms, to crowded barns of poultry. This is typically the death knell for the entire flock. If the virus is detected in a barn, “that leads to the best way that we know how to control the virus from spreading further from that barn, which is to euthanize all the birds in that barn,” said Pitesky.

Along with poultry facilities, waterfowl also spend time on the cattle farms that are concentrated in the Central Valley, which is home to the vast majority of California’s 1.7 million dairy cows. This is an emerging concern for the dairy industry as H5N1 was detected in U.S. dairy cows in late March after spreading undetected for months. It’s the first time an avian flu virus has been detected in cattle. Recently, USDA scientists linked the original spread to cattle to a wild bird. Recently, USDA scientists likely linked the original spread to cattle to a wild bird by a genetic analysis.

Scientific literature first documented waterfowl birds’ use of poultry and cattle facilities with telemetry in 2022. Led by the U.S. Geological Survey’s Western Ecological Research Center, a team of scientists, including Casazza and Pitesky, affixed a solar-powered radio signal to over 600 waterfowl that travel along the Pacific Flyway, which runs from Alaska to Patagonia.

This telemetry offered a highly precise, day-to-day account of each bird’s movements—and found that 11 birds ventured close to or directly on agricultural facilities in California and Washington. The wild birds made use of the effluent ponds, also called lagoons, which hold slurry manure. They also spent time on feedlots where cattle are fattened with grain, likely foraging for spilled grain.

“We were surprised by it,” Casazza said. “We saw species that we did not expect, like [northern] pintail and cinnamon teal, utilize some of these ag facilities.” He was only expecting this of certain species of waterfowl, like mallards, often spotted in city parks.

H5N1 has been detected in cinnamon teal, northern pintail, and mallards in the U.S., among many other species of wild birds, according to the USDA’s Animal and Plant Health Inspection Service’s data, which tracks detections of the virus, typically in dead wild birds sampled by local agencies and partners.

Casazza views this study as likely indicative of a much larger trend of birds using agricultural facilities. “They are pretty indicative of their cohort,” he said. “If one of the radios mark birds doing something, that probably means that hundreds, if not thousands of other birds, are doing that exact same thing.”

Since then, he has continued tracking waterfowl using telemetry and has observed that wild birds make use of “all kinds of alternative water features on the landscape,” including water sewage treatment plants, golf course ponds, and city ponds. Some agricultural operations can also be beneficial habitats. For instance, waterfowl utilize California’s flooded rice fields as habitat, a beneficial relationship to farmers who rely on birds to eat the rice stubble, which they used to burn.

While it remains unclear exactly what is drawing wild birds to water features on cattle and poultry farms, Casazza suggested that habitat loss may lead some wild birds to seek out alternative habitats, especially during times of drought, when their natural habitats are more constrained, a question he’d like to study. This may also lead the virus to spill over to other species, when wild birds wind up in agricultural facilities, as Casazza and other researchers proposed as a potential pathway.

It’s also a complex issue, he noted. For instance, it’s unclear whether in some cases the wild birds may be attracted to water or other resources on the agricultural facility, which may depend on the species, noted Casazza. It also may depend on whether the bird is migrating, looking for a stopover spot with water, versus moving within their wintering region—when it’s the later, he says, “when they have what they need, we tend to not see a ton of movement.” When this question was posed to Buler, he noted that it may depend on geographies too. California, for instance, is more water-restricted than the Delmarva.

As a solution, Casazza sees promise in creating more wildlife habitat for waterfowl, located away from agricultural facilities. “This would give them an opportunity to settle on habitat that is designed for them rather than these alternative habitats,” Casazza said, adding that this could also involve flooding the rice fields. He also hopes to see an increase in surveillance of waterfowl potentially as a “national data stream” that could track where wild birds are moving to help predict where the virus could be moving next.

Pitesky also emphasized the importance of more widespread waterfowl tracking, pointing to application he developed that tracks the abundance of waterfowl using remote sensing, designed as a warning system for farmers to know to implement higher biosecurity measures. For instance, he notes that farms could utilize geofencing to automatically close the curtains in the barn—used to ventilate the barn—in response to a flock of waterfowl moving toward that facility.

Another option may be that agricultural facilities relocate, so they are not clustered on waterfowl habitats. Of course, neither option is easy, especially in a place like California’s Central Valley, where nearly everyone—including farmers and waterfowl—are seeking the remaining sources of water. But there’s also rarely an easy fix to widespread infectious diseases, emerging and mutating at the complex interfaces of animals and humans, and the fragments of wildlife habitats.

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