CHICAGO – The hunt is on in the upper reaches of Lake Michigan to count what’s believed to be thousands of bird carcasses that have washed ashore this fall — a staggering toll blamed on the disruptive powers of invasive species that have taken root in the Great Lakes.

All invasive species bring consequences that few can predict, leading scientists to ponder the thousands of gulls, loons, mergansers and other migratory birds whose remains wash ashore along the beaches in northern Wisconsin and Michigan’s upper peninsula each fall.

There is a somewhat controversial theory for this annual die-off, which by some estimates has claimed more than 100,000 birds in the past 15 years, involving a type of naturally occurring but deadly botulism linked to the spread of invasive zebra and quagga mussels, which entered the Great Lakes decades ago aboard ocean vessels.

“There’s still a lot about this we don’t know,” said Joe Kaplan, of the Michigan-based nonprofit Common Coast Research & Conservation. “The one thing we do know is that it’s killing a lot of birds that are important to us.”

Zebra and quagga mussels reproduce rapidly and overwhelm their environment. Scientists feared that densely packed clusters of mussels would take a toll on industry, colonizing in water pipes, intake valves, and air conditioning and cooling systems. And they have.

The U.S. Geological Survey, which has studied zebra and quagga mussels for more than 20 years, rank them among the most destructive “biological invasions into North America.” But few could foresee the carnage that has followed.

Zebra mussels and quagga mussels filter naturally occurring botulism and other toxins from the water. Round gobies, another problematic invasive species, eat the mussels, and birds, in turn, eat the gobies.

“The evidence is there to suggest this is happening, but it’s circumstantial evidence because we haven’t found any proof of it,” said Tom Cooley, a biologist at the Michigan Department of Natural Resources. “All we can really do at this point is to continue to monitor what’s happening.”

The first sizable bird die-off count came in 1999, when researchers recorded 311 birds off the shores of Lake Erie. The following year, they found 8,000 around the Great Lakes and the death counts have remained in the thousands every year since.