Spring is in the air, with daylight savings taking effect on Sunday, and loons will begin their migration back to the north woods in less than a month.
Loons, of course, are a cultural and natural icon, not only in Minnesota but across the Great Lakes states. But last fall, nearly 900 loons died while migrating south across Lake Michigan, probably more. And it's likely at least some were from Minnesota.
Scientists are not sure what killed the loons, but they suspect that invasive species may be to blame.
In October, Lynette Grimes was hiking toward Lake Michigan at Sleeping Bear Dunes National Lakeshore, outside Traverse City, Mich. The 52-year-old from the nearby town of Benzonia has walked the beaches there for years. But she wasn't prepared for what she saw.
"The beach was just pockmarked with birds everywhere you looked," Grimes said. "This one little peninsula had over 100 dead birds."
Most of them were loons, she said, calling the sight "very, very tragic."
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Grimes and her husband worked until sunset burying the loons in 3-foot-deep trenches. Last fall nearly 600 loons washed ashore at Sleeping Bear Dunes. Scientists believe many were likely killed by the complex interplay of several different invasive species.
The scientists offered an idea about what might have happened: Invasive zebra and quagga mussels filter the water so it's incredibly clear, allowing an algae called cladophora to grow in huge amounts. Big storms churn up the algae, which settles to the lake bottom and rots. That creates an environment without any oxygen, an ideal home for bacteria that produces a deadly toxin called Type E botulism. That botulism is ingested by invertebrates, tiny worms and freshwater shrimp. And then it works its way up the food chain. They are eaten by fish, including the invasive round goby, which are then eaten by diving birds like loons.
"What happens is they can't move their muscles, and, eventually, they usually die because they can't breathe or they can't hold their head up out of the water," said Stephen Riley, a fisheries biologist with the U.S. Geological Survey in Ann Arbor, Mich.
Riley is part of a team of scientists around the Great Lakes trying to prove this theory.
Kevin Kenow, a USGS research wildlife biologist in LaCrosse, Wis., has been tracking loons with radio transmitters and geo-locator tags. Kenow's work has shown for the first time that Minnesota loons are migrating through Lake Michigan. Some spend nearly a month there fattening up before their long flight to the Atlantic or Gulf coasts.
"They're diving up to 40, 45 meters in some of these areas," Kenow said, "and the pattern of dives suggests that they aren't stopping in the water column anywhere, but they're continuing all the way down to the bottom, feeding on the bottom substrates and then returning to the surface."
And it's at the lake bottom where scientists believe fish like round gobies are picking up botulism before they are eaten by loons.
Botulism outbreaks seem to be intimately tied not just to invasive species but also to climate change, said National Park Service ecologist Brenda Lafrancois of Ashland, Wis.
"Years that are big outbreak years tend to be years that have low water levels and the highest spring and early summer water temperatures," Lafrancois said. "Those patterns have intensified in recent decades as a result of climate change."
So far large botulism outbreaks have occurred only sporadically. Before last fall, it had been five years since the previous big event. And at least to this point, they have not affected Minnesota's loon population, said Carrol Henderson, Nongame Wildlife Program supervisor at the Department of Natural Resources. The agency has monitored the state's loon population since 1994.
"The good news so far is that over the years we haven't detected any major trend in the population numbers, either up or down," Henderson said.
Minnesota has more than 10,000 adult loons, according to the USGS.
Henderson emphasized that it's still unclear how many of the loons dying in Lake Michigan are from Minnesota. But Damon McCormick, a wildlife biologist with Common Coast Research and Conservation in Houghton, Mich., is worried. Last October he found 300 dead loons in just a seven-mile stretch of Lake Michigan beach near the Upper Peninsula town of Gulliver.
"If the die-off continues, to any extent like it has, then I think it's a genuine concern for the long term viability of loons," McCormick said.
Federal scientists hope soon to pinpoint how and where botulism moves up the food chain. Then they will try to figure out a way to break a link in that chain before it makes its way to the loons.