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It's biting black flies vs. loons in the northland this season, and the flies are winning.

The flies have forced about 70 percent of nesting loons in an area of north-central Wisconsin to leave their eggs, according to Walter Piper, a professor at Chapman University in Orange, Calif. That's more than twice the highest rate of abandonment in the past 22 years, he said.

Piper first noticed the problem when he saw pairs of loons out on lakes at the same time.

"That was surprising," he said. Typically, in early May, one of the birds will be on the lake, while the other sits on the eggs. 

It turned out that a species of black fly called 

 that feasts almost exclusively on loons, was forcing them off their nests, swarming their heads and leaving large welts under their feathers. A severe winter, followed by a cool spring and then a sudden warming in early May, caused an "explosion of black flies," Piper said, "unlike anything we've seen previously." 

Male and female loons alternate incubating their eggs. If just one of the nesting pairs can't tolerate the flies, Piper said, "the other bird will ultimately abandon the nest as well, and that means the nest has failed."

The birds end up diving almost constantly to avoid the flies, Piper said, surfacing only for a few seconds before plunging below the water's surface. 

"It must be absolute misery for the loons," he said. "All they can do is shake their heads, toss their heads back against their wings, and dislodge the flies for a few seconds, before they take their blood meal, and use it to reproduce. So it's pretty nasty."

The Minnesota Department of Natural Resources says northern Minnesota loons have been leaving their nests, too. Volunteers have reported eight nest abandonments in the past two weeks, and biologists say there are likely many more.

The DNR first started receiving reports of fly-covered loons in 2011. 

"It's something that we don't really have a solution for," information officer Lori Naumann said.  

The DNR is collecting eggs from abandoned nests to test for contaminants like mercury or oil spill dispersants (left behind from the Deepwater Oil Spill in 2010 in the Gulf of Mexico, where many loons migrate in the winter) to determine if that's related. 

But there's some good news, for the Wisconsin loons anyway.

"In this past week, we've seen pairs that have abandoned their first nests owing to black flies beginning to re-nest, so many of those pairs are now sitting on eggs again," Piper said.

Environmental News

Dan Kraker

Author

Flies have forced about 70 percent of nesting loons in an area of north-central Wisconsin to leave their eggs. That's more than twice the highest rate of abandonment in the past 22 years.

Explosion of black flies forces loons to abandon nests

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