Common loons have joined whooping cranes as iconic birds under attack by black flies this year. The tiny, voracious, biting insects are driving those birds off their nests, exposing eggs to loss.

The problem is historic, but worse this year than most, involving other bird species as well.

You know about black flies if you spend time in almost any part of Minnesota near clean running water during the insects’ active period. That’s usually a few weeks in early to midsummer, the word “few” relative to the timing of your vacation or weekend visits.

Black flies are a worldwide phenomenon, with thousands of species. Some specialize by animal species in their search for blood. Loons are among birds tormented by a fly with a specific appetite.

Researchers from Northern Michigan University successfully collected these flies for examination by using loon decoys. The flies showed strong preference for visual clues. They’ll find you by detecting the carbon dioxide you exhale, or by showing strong preference for the dark-colored visual clues you might be wearing.

Dark-colored clothing is bad, light colors good if you are trying to avoid fly bites.

Female flies need a blood meal to produce eggs. Eggs are laid in streams and rivers that flow fast, have good oxygen content, and show no or little sign of pollution. (Our efforts to clean streams and rivers are being validated by black flies.) In midsummer they were in the Mississippi, Crow and St. Croix rivers as well as northern streams.

More species under attack

Minnesota has about 40 species of black fly. They are either ornithophilic — looking for birds — or mammalophilic — looking for us. They are tiny, with characteristic humped backs. Saliva numbs your skin. Razors in their mouths draw blood.

Loons and cranes make news when they abandon their nests because both species have volunteer observers paying attention. Other bird species suffer as well, only without the sympathetic observation.

Bluebirds can be hit hard. Numerous fly bites can produce toxic shock in bluebird chicks. Purple martins are attacked. Great horned owls have been known to abandon tree nests for ground nests to lessen fly attacks.

A snowy owl was found in northern Canada with both eyes completely covered with blood scabs caused by fly bites. The bare skin around eyes is a preferred target.

Red-tailed hawk chicks have reportedly jumped from nests because of attacks by black flies. Grouse are common fly targets, as are some species of gulls.

One research paper suggested that the presence of flies will cause some birds to alter spring migration routes.

Another suggested that black flies will reduce tourism in an infected area during fly season. That’s easy to believe if you’ve had the flies sneak beneath your belt or shirt collar. Black fly bites can itch forever.

On the other hand, there are species of migrant birds that fatten on flies. Swallow species and yellow-rumped warblers are mentioned in reports.

Breeding grounds

The flies made news a few years ago when the whooping crane reintroduction project based at Necedah National Wildlife Refuge in central Wisconsin found trouble. Nesting cranes early in incubation got up and walked away from their eggs. Why was a puzzle, until black flies were determined to be the villains, tormenting cranes to such an extent that the birds abandoned nests. These cranes faithfully return to the places they were raised, so cranes from Necedah return to that area regardless of flies. The birds cannot be reprogrammed.

Currently, cranes continue to get off the nests and leave. The eggs, however, are collected for incubation. Some crane pairs renest once fly season passes.

It is hoped our loons will do the same thing.

Crane reintroduction was moved east in Wisconsin, to the White River Marsh State Wildlife Area, near Horicon National Wildlife Refuge. There are few if any black flies in that area. Hopefully, cranes raised there will return to nest successfully.

Black fly numbers are amazing. Fly larvae were found 130 per square inch on debris in flowing water. Researchers estimated 1 billion — yes, billion! — hatching per day per kilometer of suitable river in season. That cloud of flies will fly as far as 15 miles to find a blood meal. After eating, female flies return to water to lay eggs for the next season.

The good news is there is only one generation per year, the flies living about a month. They like the same warm, sunny, windless days you do, but typically die when temperatures exceed 80 degrees.

They thrive when it rains a lot.


Lifelong birder Jim Williams can be reached at Join his conversation about birds at