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DAN GUNDERSON: The sun is down, but there's still gray twilight as Kevin Kenow backs a boat into South Turtle Lake, a few miles east of Fergus Falls. Three US Geological Survey researchers are here to capture two loons wearing special tags that track where they travel and how deep they dive. The crew needs to wait until the last light fades. Using bright spotlights on a dark night is the best way to sneak up on a loon, so we sit in the boat and talk while the sky darkens.

The goal of this project is to better understand loon migration. Kevin Kenow says researchers are trying to understand why so many loons die of botulism poisoning on the Great Lakes every fall.

KEVIN KENOW: Over a 10-year period, 50,000 birds succumbed to botulism on the Great Lakes. And about 50% of those were common loons. So common loons seem to be particularly hard hit.

DAN GUNDERSON: Scientists also want to learn how environmental toxins are affecting the birds. So if they catch these loons, Kenow will remove a tracking tag from their leg, take a blood sample, and clip a wing feather.

KEVIN KENOW: It looks like it's getting pretty dark. Should we see if we can catch a loon?

[ENGINE STARTING]

DAN GUNDERSON: The researchers have a general idea where the loons are on the lake, but three pairs of loons call this lake home. So the challenge is not just catching loons, it's catching the right loons. One USGS researcher drives the boat while two stand in the front, sweeping powerful searchlights across the water. After about half an hour of fruitless searching, it's time for a trick. A recorded loon call is sure to get the attention of the territorial birds.

[RECORDED LOON CALL]

[LOON CALLING]

The boat quietly turns toward the faint response. After a bit more coaxing with calls, the spotlight zero in on an adult loon with its brilliant black-and-white colors swimming alongside a dingy brown chick.

[LOON CALLING]

The boat creeps slowly forward. Kevin Kenow picks up a large fishing net and leans over the front of the boat.

[WATER SPLASHING]

He scoops the adult up and puts it in a crate. It's the male of this loon family. This is one of the birds researchers are looking for. It's wearing the leg band they put on a year ago. After about a 10-minute search, they get the small brown-colored chick, as well. But they can't find the female.

Kenow decides to head back to shore and get information from the male and the chicks so they can be released. This lake is just one stop for these researchers. They're on a three-week tour of lakes across Minnesota and Wisconsin. In the past three years, they've captured and tagged a total of 176 loons. Back on shore, the male loon protests as he's examined.

[LOON MOANS]

Everyone tries to stay clear of the spear-like black bill.

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

DAN GUNDERSON: Kenow looks at the tag on the loon's leg and finds a problem. The small tube holding the data recording device that records the loon's travels is gone. Kenow says there appears to be a flaw in the glue holding the geolocator to the leg band. An entire year of data about where this loon traveled and how deep it went to catch fish is lost. It's a new problem this year, and Kevin Kenow is frustrated by the technological failure.

KEVIN KENOW: It's lost information. They warrantied we'll get replacement tags, but a lot of effort goes into capturing the bird and recapturing the bird.

DAN GUNDERSON: This loon won't get a new tracking device since this phase of the study is ending, but researchers can still get some useful information from this loon using a tiny needle. Kenow draws a blood sample, and 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 or mercury and other contaminants.

After raising their chicks, adult loons leave Minnesota in the fall to winter on the Gulf of Mexico. But first, they head east and spend about a month on the Great Lakes eating fish and preparing for the long migration. What they eat sometimes kills them. Kevin Kenow says tracking loons' migration and behavior is helping to unravel that mystery.

KEVIN KENOW: What we've learned about loons and their use of, for example, Lake Michigan is that they feed further offshore than we had previously thought. And they repeatedly dive to depths of 40 to 50 meters.

DAN GUNDERSON: That's about 150 feet, which means loons are feeding on fish near the lake bottom. That 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.

KEVIN KENOW: We know from stomach content analysis that some of the loons that wash up on shore contain gobies. So gobies might certainly be involved in this cycle.

DAN GUNDERSON: Kenow puts numbered and colored leg bands on the male and the chick. Then it's back in the boat to return the loons to where they were captured. Kevin Kenow initiates a quick conversation with the loon as it swims into the dark lake.

[WATER SPLASHING]

KEVIN KENOW: Hoo-hoo.

[LOON HOOTED BACK]

DAN GUNDERSON: They find the missing female loon, and the whole process begins again. It's after 1:00 AM when they released her. These researchers have two more lakes to search before dawn. They hope this effort will pay off with a better understanding of loons and the natural and environmental risks the iconic birds face. Dan Gunderson, Minnesota Public Radio News, on South Turtle Lake.