Since 2006, dead birds have been washing up on Lake Michigan beaches.
But this fall has been exceptionally grim, with up to five thousand birds being found dead along the shore.
Researchers are trying to understand what's happening.
IPR's Sam Corden reports....They're walking along a beach in Sleeping Bear Dunes National Lakeshore. The typically pristine coast is littered with about two dozen dead birds including scoters, loons, ducks, and more.
Researchers say the birds are dying because of a toxin called avian botulism, which can form on the lake bed under certain conditions.
Standing over a dead duck, Ray describes what he sees, and the procedure that follows.
"So we have a long-tailed duck, and we're going to pick that up away from the shoreline, take it up into the foredune," Ray says. "And then we dig a hole two feet deep, and bury it so that it's away from park visitors and pets and no longer a threat to public health."
Botulism forms when there's a lack of oxygen in the water.
Ray digs a hole to bury a dead bird. Image: Samuel Corden
For that to happen, it takes a long chain reaction that begins with invasive Zebra and Quagga mussels filtering the water, which increases its clarity.
Williams of Inland Seas explains.
"The mussels are eating all this stuff, and then they produce waste like all living things do, and that waste has all kinds of nutrients in it … lots of nitrogen, lots of phosphorous," Williams says. "And because there's a lot of nutrients at the bottom of the lake, and a lot of sunlight, that means the algae can grow as much as it possibly can."
The native algae thrives on those nutrients and the additional sunlight, growing prolifically over the summer months.
However, living algae poses no issue, and it's not until it dies that it becomes a problem.
Harvey Bootsma is a professor at the University of Wisconsin Milwaukee. He says the northern part of Lake Michigan is the perfect place for botulism to thrive.
"One thing that's unique about the north end of the lake, especially in the Sleeping Bear region, is that the bottom of the lake, the lake topography, or what we call bathymetry is quite variable up there," Bootsma says, "so you have a lot of these pockets in the bottom of the lake … little valleys that are formed all over the place."