Walter Piper, a professor at Chapman University in Orange, California, says the flies have forced about 70 percent of nesting loons in an area of north-central Wisconsin to leave their eggs. He says that's more than twice the highest rate of abandonment in the past 22 years.
Piper tells Minnesota Public Radio News a severe winter, followed by a cool spring and then a sudden warming in early May, caused an "explosion of black flies."
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.