Antibodies Suggest Some Raptors May Be Developing Resistance to Avian Influenza

From the Spring 2026 issue of Living Bird magazine. Subscribe now.

A Great Horned Owl quietly sits in her dim-lit quarantine crate in the clinic of The Raptor Center at the University of Minnesota in St. Paul, staring off into space following her intake exam. Found in the middle of the road just outside of the Twin Cities in April 2022, the owl was presumed to have hit her head after being side-swiped by a car. She seemed perfectly normal, otherwise—certainly not infected with a raptor-killing virus.

So it was a surprise to Dr. Dana Franzen-Klein, the center’s medical director, when test results for highly pathogenic avian influenza, or HPAI, came back positive on the owl.

“I’d never seen [an HPAI infection] that mild before,” says Franzen-Klein. “She didn’t have any of the severe signs, like seizures.”

The Great Horned Owl made a full recovery, from her injuries and HPAI, within a few weeks. But most raptors admitted to the center that year, at the peak of the 2022 North American HPAI outbreak, did not share this owl’s fortunate fate. Out of the 213 admitted raptors that tested positive for HPAI that year, she was the sole survivor.

However, new research suggests more wild raptors may be surviving the disease than previously thought. A study led by staff at The Raptor Center and published in February 2025 in the journal Scientific Reports found that 23% of tested raptors in Minnesota carried antibodies against HPAI. Of those birds, 70% had antibodies specific to the H5N1 strain that has ravaged North America in the last few years.

Antibodies indicate that the bird had caught the disease in the past and survived it, boosting its immune system against reinfection. Franzen-Klein says antibodies for avian influenza had previously rarely been detected in raptors. Before this study, many researchers had assumed most birds were dying from H5N1, but the Great Horned Owl’s survival gave veterinarians pause.

“Her story was what spawned this whole project,” says Dr. Kelsey Rayment, a former center veterinarian who spearheaded the antibody study. “Because we were like … ‘If one could survive here, could there be others surviving out there?’”

Avian influenza has existed in different forms around the world since the 1800s, rising and falling with bird migration as infected birds carry the virus across continents. In recent years, the disease has caused numerous mass die-offs of wild birds—from Snow Geese in Pennsylvania to Common Cranes in Germany—and spread to mammals like red foxes in North America. About 20,000 HPAI cases in birds and mammals have been confirmed in the United States since 2022.

At the start of the outbreak in Minnesota in 2022, The Raptor Center’s veterinarians took on an onslaught of HPAI-ridden patients. At the time, the vets thought most or all of these birds were unlikely to make it. Franzen-Klein handled most of the HPAI patients that year, and she says many were gravely ill, stumbling about and having seizures. HPAI causes inflammation in the brain, and in many cases, putting the bird to sleep was the only humane option.

“It’s very, very hard to deal with that,” says Franzen-Klein, “and see birds suffering.”

Rayment joined the triage team at the center near the tail end of the spring 2022 peak. She and other veterinarians wondered if there were milder cases that never reached the clinic. 

“We’re only seeing really sick birds,” she says.

So Rayment started the project to collect blood samples from center patients and a selection of wild raptors captured for research and banding at Hawk Ridge Bird Observatory in Duluth, Minnesota, about two hours’ drive north of the Twin Cities. After analyzing the samples, her team made the unexpected discovery of flu antibodies in many birds.

Rayment was particularly impressed by the large proportion of Bald Eagles that had antibodies, with 54% of tested eagles showing antibodies to H5N1 HPAI, and 15% to other avian influenza strains. To Rayment, these numbers show just how many raptors were surviving the disease.

“When you come in and you’re seeing all these birds really sick and dying,” Rayment says, “the fact that we were able to see that some had survived was a little bit of hope for the future resilience of the population.”

Since then, the number of positive cases of the disease has declined, although HPAI reappears in birds and mammals every fall through early spring. The Raptor Center saw 213 confirmed positive cases in 2022, but only 10 in 2023 and 14 in 2024. Fourteen cases in 2025 have been confirmed positive, while 20 more are in the process of confirmation at an off-site lab.

Rayment and Franzen-Klein agreed that antibodies may be one contributor in the reduced number of sick raptors showing up at the center.

“It is likely that there is now more immunity in the raptor populations, contributing to the decline in cases,” Rayment says, “which we could see in the antibodies detected in this study.”

The 2022 North American HPAI outbreak was particularly nasty when compared to previous outbreaks.

“This is the only outbreak, at least in recent years, that has caused this level of morbidity and mortality in wildlife species,” says Dr. Sara Childs-Sanford, director of the Janet L. Swanson Wildlife Hospital at Cornell University. “We don’t typically see raptors being affected to this level.”

The H5N1 strain of HPAI also has a unique staying power, having hung around for multiple years. Previous HPAI outbreaks have blossomed during migration season before disappearing once birds spread out for breeding. H5N1, however, is on its way to becoming an endemic virus—like the common cold in humans, only much more deadly.

Neil Paprocki was a PhD student at the University of Idaho when HPAI swept the continent in 2022. He estimates that more than a quarter of North America’s Rough-legged Hawk population may have died at the start of the outbreak.

Paprocki tracked 71 Rough-legged Hawks with GPS devices throughout the outbreak. He recalls watching GPS-tagged hawks go down like flies in the few months that the virus was at its peak. Each time a tagged roughleg went down, local game wardens were sent to recover the bodies and test for the cause of death.

“I suspected after a couple birds what was going on, but it was just really overwhelming seeing bird after bird go down,” Paprocki says. “It was very confusing, frustrating, scary.”

Between March 2022 and March 2023, 29 of the tracked hawks died or were presumed dead. Of those, 11 deaths were confirmed to be from HPAI, and another nine were likely, though unconfirmed, HPAI cases. The overall 47% mortality rate in 2022 was far higher than the normal range of annual deaths—between 3% and 17%—that Paprocki had observed while studying roughlegs in previous years.

Since 2022, however, HPAI-related mortality among Rough-legged Hawks has dramatically fallen. Paprocki only saw one H5N1 mortality in his study cohort in 2024, and none in 2025. He believes this decrease may be due to the virus itself growing weaker.

“If the virus was as virulent as it was [in 2022], some of these younger birds could still be getting exposed to the virus and have pretty high mortality rates,” says Paprocki, who is now a postdoctoral researcher at the Cornell Lab of Ornithology. “We know the virus is still out there, but it doesn’t seem to be out there at the level that it was in 2022.”

Researchers at The Raptor Center in Minnesota are still grappling with a slew of new questions about HPAI.

“We don’t exactly know why some individuals survive infection and some don’t,” Franzen-Klein says. 

The Raptor Center is continuing to diligently collect blood samples and test for antibodies in clinic patients and in Hawk Ridge’s banded birds.

“It’s really hard to say what’s going to happen in the future,” says Franzen-Klein, “because this virus could change in a way where it makes birds sick again and we have another major die-off.”

As for the feisty Great Horned Owl that earned the title of sole survivor during the peak of the HPAI outbreak in spring 2022, she was banded after recuperation at The Raptor Center and released in a public park in May 2022 near where she was originally found. Her band has not been recovered since.

“Hopefully,” says Franzen-Klein, “that means she’s still doing well.”