Under cover of darkness, researchers capture, track loons in Minn.

How do you sneak up on a loon?

That's the question this night as wildlife scientists slide a boat into South Turtle Lake, a few miles east of Fergus Falls.

The biologists want to know why so many of the iconic birds die of botulism poisoning on the Great Lakes every fall. They want to learn more about environmental toxins loons face on their long annual migration.

But first they have to catch them.

On the lake, they seek a pair wearing special tags that track where they travel and how deep they dive. A recorded loon call gets the attention of the territorial birds. After a bit more coaxing with calls, the spotlights find an adult loon, in brilliant black and white, swimming alongside a dingy brown chick.

Click for photos of the late-night loon capture

• MAP: Track loons' migration paths

Scooping the adult into a fishing net, U.S. Geological Survey wildlife biologist Kevin Kenow sees it's the male of this loon family, wearing the leg band researchers put on a year ago. He snags the small, brown-colored chick but can't find the female. He decides to head back to shore and get data from the male and chick so they can be released.

South Turtle Lake is one stop on a three-week tour of lakes across Minnesota and Wisconsin. The states boast the greatest concentration of nesting loons in the U.S. Adult loons leave here in the fall to winter on the Gulf of Mexico after raising their chicks. But before going south, they head east for a month on the Great Lakes, eating fish and preparing for the long migration.

What they eat there sometimes kills them. Botulism on the Great Lakes has killed 50,000 birds in a decade — half of them loons, Kenow said. Tracking the loons' migration and behavior, he added, is helping unravel the mystery.

Back on shore, the male loon protests as he's examined. Everyone tries to stay clear of the spear-like black bill.

"Every summer they draw blood at some point," Kenow said. "They can stab or when they grab your finger it's like having a vise grip on it. If you pull away it will just rip your skin."

COLLECTING DATA A TOUGH TASK

Kenow looks at the tag on the loon's leg and finds a problem. The small tube holding the data recording device that records its travels is gone. Kenow says there appears to be a flaw in the glue holding the geo-locator to the leg band.

An entire year of data about where this loon traveled and how deep went to catch fish is lost. It's a new problem this year and Kenow is frustrated by the technological failure. "A lot of effort goes into capturing the bird and recapturing the bird."

Researchers, though, can still get some useful information.

Using a tiny needle, Kenow draws a blood sample. He also clips off a wing feather. The blood and the feather can tell scientists if this loon was exposed to residue from the BP oil spill in the Gulf of Mexico in 2010 or mercury and other contaminants.

On Lake Michigan, the data revealed loons feed farther off shore than previously thought, with the birds diving repeatedly to depths of about 150 feet.

That means they're feeding on fish near the lake bottom, which might make them susceptible to toxic botulism that grows in the algae there.

One theory is that an invasive fish species called the round goby carries the botulism.

"We know from stomach content analysis that some of the loons that wash up on shore contain gobies. So gobies certainly might be involved in the cycle," Kenow said.

He puts numbered and colored leg bands on the male and the chick, one of 176 loons the team has captured. Then it's back in the boat to return the loons to where they were caught.

In that journey back, they find the missing female loon. Kenow initiates a quick conversation with the male as it swims into the dark lake. He clucks and the loon answers.

It's after 1 a.m. when they release his mate. These researchers have two more lakes to search before dawn.

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