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^{Common Loons washed ashore in October 2012 along a seven-mile stretch of Lake Michigan beach near Gulliver, in Michigan's Upper Peninsula. Photo courtesy of Damon McCormick/.}

In late September, the body of a 21-year-old male common loon washed up on a Lake Michigan shore. A band around its leg revealed a rich history: The bird was the last of a group of 13 loons marked between 1991-1993 in Michigan’s Antrim County, in the Lower Peninsula. It had logged some 42,500 miles and fledged 16 chicks over the course of 12 years. Other than a broken beak tip, the lifeless bird appeared to have been healthy enough, weighing a solid 10.2 pounds, according to a necropsy report. Further tests revealed the cause of death: a strain of avian botulism called , which has been plaguing loons and other waterbird species in parts of Lake Michigan for the past six years.

In 2007, more than 4,000 affected birds from Lake Michigan beaches were reported dead, according to a on Type-E botulism outbreaks. Monitoring at various locations around the lake during this past summer and fall suggest another grim year. Around 1,444 sick and dead birds—including red-necked and horned grebes and white-winged scoters—were collected from Michigan’s (in the northwestern part of the Lower peninsula) alone, for example; about 580 of those were common loons. In fact, the month of October saw the heaviest loss in that species since rigorous record keeping began at the park in 2007.

“From a loon perspective, the numbers are really quite large,” says Damon McCormick, a biologist and co-director of the nonprofit organization , which does bird research and conservation work (with an emphasis on common loons).

A long-lived species with a low reproductive rate, common loons are listed as threatened in Michigan, probably numbering fewer than 1,000 pairs. “The loon is kind of like the grizzly bear or the bald eagle or the wolf—they’ve all got kind of an aura about them,” says Thomas Cooley, a wildlife biologist and pathologist at the Michigan Department of Natural Resources' , “People identify them with wilderness.”

While tests on the birds collected from Sleeping Bear Dunes and other locations aren’t yet complete, preliminary results, as well as past evidence, point to Type-E avian botulism as the primary culprit.

^{Common Loons washed ashore in October 2012 along a seven-mile stretch of Lake Michigan beach near Gulliver, in Michigan's Upper Peninsula. Photo courtesy of Damon McCormick/.}

Such botulism outbreaks, which have also been documented in several other lakes in the system over the past decade, seem to require a perfect storm of sorts. “It’s probably a concert of drivers all at once” says Jennifer Chipault, a wildlife biologist at the who oversees an avian botulism monitoring volunteer program in Wisconsin. “We’re trying to parse it apart as much as we can.”

One theory suggests that the toxin makes its way into birds like this: Zebra and quagga mussels—invasive species—filter lake water, making it clearer and allowing more sunlight through, which spurs algae blooms that grow into blanket-like mats that slough off onto the beach. As the mats decompose, they deplete oxygen from areas on or near the shore. Anaerobic conditions, combined with warmer temperatures and nutrients released from decaying organisms, can then drive a native bacterium to release the Type-E botulism toxin into the water.

Though researchers aren’t sure of the mechanism, it’s possible that as invasive mussels filter the water, they concentrate the toxin—which might subsequently be consumed by birds feeding directly on the mollusks. Fish can also collect the toxin, putting seafood-eating birds at risk. The consumed toxin, in turn, affects the birds’ nervous system, paralyzing muscles, such as in their neck. Unable to hold up their heads, they can drown and later wash up on shore. 

How to manage the epidemic is far from clear. Removing all of the invasive mussels is “probably not realistic,” says Chipault. Instead, mapping out hotspots where the toxin is prevalent might be helpful—wildlife managers could ward birds off from those places during migration. Cleaning up contaminated and decaying algae mats that have washed ashore is another potential tactic, says Chipault.

Meanwhile, protecting loon habitat from other threats, such as development and boat traffic, is an important conservation measure. (a Michigan Լ����Ƶ project), for instance, works on land acquisition projects, monitoring, and public outreach in a 500-plus-acre watershed in the northwest part of the state. Says Margaret Comfort, a self-professed loon lover and the network’s project manager. “We do what we can to help the [loons] that are here and to maintain the habitat so it will be available.”