The spread of highly pathogenic avian influenza (HPAI) across North America in 2022 has taken an unprecedented toll on wild birds, according to federal data.
Already the number of birds dead from the virus is more than 10 times greater than documented in any other avian flu outbreak.
But as with many wildlife diseases, the impacts of HPAI are difficult to assess as wild birds often disperse and die never to be found.
A sobering exception was discovered in early June by researchers visiting Caspian tern breeding sites in Wisconsin.
Where they expected to find colonies of the state endangered bird brooding eggs, they instead saw windrows of dead adults.
The two islands off Door County — Gravel Island and Hat Island — had 1,221 dead Caspian terns.
Many other birds were nearly dead and displayed neurological impairment including head tremors, head tilting, loss of coordination and were unable to walk or fly properly, according to a report by Sadie Odell of the U.S. Fish and Wildlife Service.
The symptoms are consistent with HPAI.
One adult tern on Gravel Island was observed in an incubating posture next to a nest and unable to move, said Sumner Matteson, avian ecologist with the Wisconsin Department of Natural Resources, of his June 2 visit to the site.
Nearby a Herring gull chick sought shelter under the shade of a dead tern.
The dead Caspian terns on the islands represent the loss of at least 64% of Wisconsin's breeding population of the species, according to Matteson.
"Just devastating," Matteson said. "The most horrific thing I've witnessed in my 42 years doing bird work."
Lesser numbers of dead double-crested cormorants and Herring gulls were also seen on the islands.
Necropsies on seven Caspian terns and two cormorants from the islands were conducted at the National Wildlife Health Center in Madison.
All of the submitted birds from both islands were positive for HPAI, said Nancy Businga, DNR wildlife disease specialist.
The case highlights how significantly the virus can impact wild birds.
The 2021-22 outbreak of avian flu in North America is already the worst in seven years among U.S. domestic flocks and the deadliest ever documented among America's wild birds, including raptors and waterfowl.
The highly pathogenic avian influenza (HPAI) strain, called EA H5N1, has been circulating since late 2021. It had been detected in 42 states as of June 24, according to the U.S. Department of Agriculture.
So far 40.1 million domestic birds have died in the outbreak, mostly through depopulations at infected poultry farms, according to federal figures.
It's the biggest since 2015 when HPAI subtype H5N2 affected about 49 million domestic birds. At a cost of $1 billion, the outbreak was the most costly animal health emergency in U.S. history, according to the USDA.
In the last major avian flu outbreak, 98 HPAI-positive wild birds were detected between December 2014 and June 2015, according to the USDA. Most were hunter-harvested waterfowl from the Pacific Flyway.
That number has spiked to 1,585 among wild birds in the current outbreak, according to USDA figures released June 24.
But as evidenced by the Caspian tern findings in Wisconsin, the number of dead wild birds in the federal report vastly underrepresents the toll of the disease.
The global spread of avian flu is driven in large part by migratory waterfowl, according to the USDA.
Knowledge about migratory patterns and intercontinental associations of waterfowl, as well as genetic analyses of viral strains, supported the hypothesis that previous HPAI outbreaks entered North America from Asia via migratory birds.
Wild waterfowl and other species shed the virus into the environment through their oral and nasal secretions and feces.
Scavenging and predatory species can also pick up the virus by feeding on infected dead or live birds.
The disease has affected dozens of species, including bald eagles, red-tailed hawks, Canada geese, lesser scaup and American white pelicans.
Songbirds appear to be relatively unaffected.
But many researchers are concerned about the impact the disease will have on wild bird reproduction and recruitment this year.
Madison Audubon has conducted a Bald Eagle Nest Watch program since 2018 in Wisconsin. The citizen-science effort includes frequent monitoring of eagle nests from the mating season in winter through fledging in spring and summer.
From 2018-21, the program recorded 28 eagle nest failures on 187 nests observed, or 15%. The annual failure rates ranged from 9% to 28%.
This year the failure rate has spiked to 46% on 145 monitored nests, according to data provided by program organizer Drew Cashman.
Eagles in the Fox River Valley fared worse than other areas in the study. Nest failures there reached 71% (17 of 24).
Dead chicks were not tested for disease and it's not possible to attribute all the failures to HPAI. But in one case, an adult eagle in Milwaukee County found unable to fly near a nest and later euthanized tested positive for HPAI.
Colonial nesting waterbirds are being hit hard by the disease. Among the Wisconsin death toll attributed to HPAI are 1,143 dead double-crested cormorants, 106 American white pelicans and 71 Herring gulls, according to data provided by Matteson.
The tally includes 202 dead cormorants and 43 pelicans at Horicon National Wildlife Refuge near Horicon.
It's not clear why Caspian terns have died at even higher rates than the others.
A researcher in Michigan found similar devastation as to what occurred at the Wisconsin islands. Caspian tern colonies were "wiped out" on Bellow Island in Grand Traverse Bay with 255 dead adults, according to a Michigan Radio report.
Conversely, common terns in the region are faring much better, Matteson said. It's possible differences in the migration patterns and habits of the two species will provide answers.
For now Matteson said he and the other researchers were left with an overwhelming feeling of helplessness.
Caspian terns can live to 30 years of age and don't breed until they are 3. The disease has not only killed a significant number of the adult breeding population this year but will likely result in very few, if any, chicks that survive.
Matteson said it will likely take decades for the Caspian tern population to recover from this year's die-off.
"I've seen a lot in my career, but this is the worst," Matteson said. "It's absolutely catastrophic."
Researchers remain hopeful that, similar to previous HPAI subtypes, the virus will dissipate this summer and not spark another outbreak.
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This article originally appeared on Milwaukee Journal Sentinel: Avian flu has taken a toll on dozens of wild bird species in 42 states