The bird die-off was obvious as soon as Gary Rentrop reached the Lake Michigan shore. The sugar-white sand, long buried in the crushed gray shells of invasive mussels and mats of rotting algae, was now littered with dead birds.
“It was almost like a war zone of birds,” said Rentrop, a lawyer who recalled his November stroll along a remote beach near Cross Village, Mich.
Rentrop counted 80 carcasses in a mile. That was just a fraction of the estimated thousands of dead mergansers, gulls, loons and other birds whose migration last autumn ended in deadly poisoning from Type E botulism on Lake Michigan.
The mounting toll on migrating birds has stoked fears among researchers and ecologists that the deaths happened because of invasive populations of zebra mussels and round gobies over the Great Lakes, which effectively create a new food chain.
Zebra mussels and their deep-water kin, quagga mussels, filter naturally occurring botulism and other toxins from the water. Gobies eat the mussels, and birds, in turn, eat the gobies.
Scientists theorize that this new food chain is concentrating botulism and other bottom toxins and passing them up to predators. The theory is the subject of a handful of scientific papers and upcoming research proposals.
Whatever the mechanism of transmitting the botulism, scientists in 1999 counted 311 birds in Lake Erie that appeared to have died from it. The next year they counted 8,000, and the number has remained in the thousands in the Great Lakes every year since. Instead of fading quickly, as sporadic outbreaks did in decades past, the toxin has spread -- first through Lakes Erie and Ontario, then Huron.
In 2006, Lake Michigan was the most recent lake to be affected and by last autumn was one of the hardest hit.
In spreadsheets, scientists have noted the fatal effects of the annual outbreaks on more than 50 species of birds throughout the Great Lakes, from bald eagles to pigeons. The list names 16 species of ducks, four types of grebes and six types of gulls. It includes double-breasted cormorants and four of Lake Michigan’s tiny piping plovers, a bird so threatened that its nests are protected by police tape and fences at Sleeping Bear Dunes National Lakeshore.
The deaths of many hundreds of loons have focused new urgency on the now-annual die-offs. Loons, a symbol of northern wilderness, live in small numbers and are slow to reproduce.
The die-off that ended in November claimed an estimated 3,500 to 8,500 birds over hundreds of miles of beach in seven northern Michigan counties.
It had spread from an estimated 2,900 birds in 2006 along just 14 miles of shoreline at Sleeping Bear Dunes, said dunes wildlife biologist Ken Hyde.
The die-off also sparked preparations for a sprawling and macabre bird count in 2008 that will involve scores of volunteers combing hundreds of miles of Lake Michigan beaches over the summer and fall -- to add up, bury and haul away what are expected to be thousands more poisoned birds and fish.
“We wish we weren’t dealing with this,” said Mark Breederland, extension educator for the Michigan Sea Grant, who is organizing the upcoming response. “We’ve got enough challenges on Lake Michigan, but it’s here. It’s upon us.”
The heightened threat to Lake Michigan became clear over the summer, when shore birds began dying, possibly from picking maggots off infected fish carcasses that washed ashore.
Then came autumn. “We were getting so many loons,” said Thomas Cooley, a Michigan Department of Natural Resources biologist who performed necropsies on the birds. It takes 10 or 12 of the big birds to cover a laboratory table, he explained. “When you have two or three tables covered with those, it’s pretty sobering to look at that.”
Among the birds found dead was one of the most studied loons in Michigan, a venerable male with four boldly colored tags on his legs and a name: C-3. Each year since 1993, he had been observed at an Upper Peninsula pond in the Seney National Wildlife Refuge, said Damon McCormick, a wildlife biologist at Common Coast Research and Conservation who studied the bird.
Researchers knew that C-3 had spent much of his life with the same female loon on a secluded pond in a corner of the refuge and that for unexplained reasons, he had recently left her for another loon on a neighboring lake.
They knew that he stayed behind a few weeks this year to supervise one late-blooming chick as other loons began their fall migration, which may have timed his migration perfectly to a botulism plume and indirectly spelled his doom. To their knowledge, C-3 had raised more than 15 chicks over the years, and only once let a chick drown -- when its leg got caught on a submerged log -- a record that made him a good father, researchers said.
The loon’s body was found Nov. 1 by an old friend of sorts, on a deserted, sandy crescent of Lake Michigan’s north shore.
Biologist and Common Coast founder Joe Kaplan had handled C-3 “four or five” times in 14 years, most recently in 2006. Kaplan was on his last day of surveying bird carcasses along the shore when he discovered the dead loon.
“I remember specifically walking up to this bird,” Kaplan said. “There are thousands of thousands of birds that died on that lake, and here’s a bird that had a known history. I had a relationship with this bird. It’s an element of familiarity that you didn’t want to find.”
Adult loons return to their northern nesting grounds by early spring about 93% of the time, McCormick said. This year, researchers will be watching for them anxiously. A decline in adult population would almost certainly spell a decline among loons.
“We expect to see all our birds,” McCormick said. “But based on finding the C-3 male, there’s a lot more trepidation of what we’ll find this spring.”