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Bird flu has killed nearly 1,500 Caspian terns on Lake Michigan islands

birds flying
Caspian terns are the largest terns in the world. They migrate to the Great Lakes region to nest. (Courtesy of U.S. Fish and Wildlife Service)

Bird flu is the likely culprit in the deaths of nearly 1,500 Caspian terns, black-crowned seabirds that feed along islands in Lake Michigan. 

Wildlife biologists are finding whole colonies of birds dead or dying on the islands. Caspian terns are listed as threatened in Michigan and endangered in Wisconsin.

This article is part of The Great Lakes News Collaborative, which includes Bridge MichiganCircle of BlueGreat Lakes Now at Detroit Public Television, and Michigan Radio. It unites newsroom resources to report on the most pressing threats to the Great Lakes and drinking water supplies, including pollution, climate change, and aging infrastructure. The independent journalism is supported by the Charles Stewart Mott Foundation.

“Seeing hundreds of dead birds scattered in a line before you with others dying among those…it's a feeling of helplessness, knowing that there's nothing, absolutely nothing you can do for those birds,” said Sumner Matteson, an avian ecologist with the Wisconsin Department of Natural Resources. 

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Matteson said he’s never seen anything so traumatic in his 42 years on the job.

On Bellow Island in Michigan’s Grand Traverse Bay, another scientist discovered colonies that were wiped out. 

“Last count prior to the time we were out there was 201 nests and we found 255 dead adults,” said Jim Ludwig, an environmental consultant who has studied Great Lakes birds for decades.

dead birds scattered on a beach
In the distance, a worker is picking up dead Caspian terns. Hundreds of them died from bird flu on this Wisconsin island. (Courtesy of Sumner Matteson, WI DNR)

He guessed that more than that died elsewhere, including in the lake, while scavengers might have carried off other remains.  

Found along the coasts of oceans and large lakes, the crow-sized birds are white and gray in color, with long, reddish-orange, black-tipped bills. Long-lived and slow to procreate, they can survive 30 years but don’t breed until they’re at least three years old. The terns eat mostly fish, with a particular taste for alewife in the Great Lakes, and nest in dense ground colonies, which could make them particularly vulnerable to spreading disease.

“Caspian terns are magnificent birds. They've got that striking black cap and they fly along, looking down at the water while they fly and then suddenly plunge into the water to catch fish. They're exciting to watch,” said Lisa Williams, a Contaminants Specialist with the U.S. Fish and Wildlife Service.

line of dead birds
A line of dead Caspian terns with their exposed nests full of eggs in the foreground. (Courtesy of Sumner Matteson, WI DNR)

In recent years, the bird’s population has been growing. In 2018 they peaked at about 10,000 Caspian terns in the Great Lakes region. Then high water levels made nesting difficult for the birds.

Now it appears that Highly Pathogenic Avian Influenza known as bird flu is killing them by the hundreds.

“Caspian terns nest very close together,” Williams said. “And for a disease that's transmitted through the air, they're in close enough proximity that that can happen fairly readily on their colonies.” 

The result is that at least 1,476 adult terns are dead on Lake Michigan islands

chick under wing of dead bird
A Herring gull chick finds shelter under the wing of a dead Caspian tern. (Courtesy of Sumner Matteson, WI DNR)

Sadie O’Dell, a wildlife biologist at Wisconsin’s Gravel Island National Wildlife Refuge, a protected breeding ground for colonial nesting birds, said the terns discovered still alive could barely hold their heads up. They were experiencing tremors from the neurological damage caused by the bird flu virus. Some were in their nests, still trying to incubate eggs when they died.

Matteson said at this point, as estimated 64 percent of the adult Caspian terns in Wisconsin are dead.

“Absolutely devastating. Catastrophic. It's going to take years for the Wisconsin population to recover,” he said, or perhaps decades. 

Such a massive die-off will mean the loss of a new generation of Caspian terns, said Francie Cuthbert, a professor with the University of Minnesota’s Department of Fisheries, Wildlife and Conservation Biology.

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“No young are being produced,” Cuthbert said. “And then the loss of all of these adults is serious.”

“Losing all these older, experienced breeders is also very important because they tend to increase in terms of their productivity and just their knowledge of how to raise young,” Cuthbert said. 

 Why Caspian terns are being hit so hard by avian influenza, while other close-nesting seabirds have not experienced the same kind of devastation is baffling to the scientists. There have been deaths among ring-billed gulls, cormorants, and others, but not at the rate of Caspian tern deaths.

Cuthbert said it’s hard to say how critical the deaths will be to the future of the bird. “Until we really have a full tally on how many birds have died, we're not going to be able to model the impact on the population, but it's definitely going to have a deep drop.”

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