Owl irruption winds down, tracking population trends in Superior National Forest

As the snow fades from northern Minnesota, so does the influx of owls that migrated into the area during the latest owl irruption event.

Irruptions occur when there is a sudden change in the population density of a bird species across a broad geographical range. The cycle occurs every three to five years and is driven by various factors, including food, weather, or other environmental conditions.

Now that spring has arrived, and the snow has started to recede from the landscape, owls will begin venturing north again to start the breeding season.

“By March, some of these species are starting to pair-bond, and then a lot of them will start actually laying eggs and incubating in April,” said Dave Grosshuesch, a wildlife biologist with the U.S. Forest Service based at the Tofte Ranger Station. “Especially when we have these warm, kind of southerly winds coming, they may start moving back north. It all is going to be dependent on the body condition of the bird.”

Great Gray Owl | Photo by Jean Brislance

While irruption events are highly variable in the number of bird species and the location or duration in which they stay, some noticeable trends have emerged as the 2024-25 owl irruption winds down.

Grosshuesch said the first noticeable trend is that “this irruption, this year, is probably the largest irruption we’ve seen in about two decades.” The largest owl irruption in recent decades was in 2004-05. During 2004-05, there were thousands of recorded sightings of owls migrating south. Grosshuesch said that the 2024-25 irruption occurred in the “hundreds of owls rather than thousands.”

Another trend Grosshuesch sees as the owl irruption tapers off is that the 2024-25 irruption consists of adult Great Gray Owls rather than younger ones.

As part of his ongoing research, Grosshuesch bans owls in the Superior National Forest to assess body condition, take measurements, document age and sex, and other helpful information to understand the primary causes of an irruption. He also collects dead owls who have died of natural causes, diseases, or succumbed to a vehicle or window collision.

“This year, at least for Great Grays, I have not seen or caught or heard of any young birds being found,” Grosshuesch said. Furthermore, he said he had noticed a similar trend for Boreal Owls. “So no birds that were born this last year or this last summer are or have been found at this stage for Boreals.”

Instead, Grosshuesch said he is documenting a “large number of second-year birds.”

He said while many variables can contribute to the trends he is documenting, the lack of younger birds may be attributed to a previous low reproduction year, potentially driven by lack of food or poor body condition. “If they’re not in a good enough body condition, they just won’t breed.”

While there has been an influx of owls recently, Grosshuesch said the overall population health of owls in the Superior National Forest is “pretty good.” However, one owl species has shown a decline following recent surveys.

The three most common owl species in the Superior National Forest are the Barred Owl, the Northern Saw-whet Owl, and the Great Horned Owl. The Great Gray Owl and the Boreal Owl also frequent the national forest and regularly nest.

Grosshuesch said that the population of Great Gray Owls is “pretty stable” despite being considered a sensitive species. As for the Great Horned Owl, Grosshuesch said populations “actually seem to be increasing slightly.”

However, Grosshuesch said that populations of the smaller Boreal Owl are not experiencing an increase in numbers. “We have seen a decline in Boreal Owls. And it’s not just something that has been witnessed here in Minnesota. It’s even in the Canadian provinces.”

Boreal Owl | Photo by Jean Brislance

Grosshuesch said that it is “really difficult to know why” populations are declining at this time.

In the upcoming spring Superior National Forest owl survey, Grosshuesch said the goal is to continue monitoring Boreal Owls and identify nesting areas to prohibit management activities. “Or if we do, we would limit when we are going to do any type of activities so that we’re not disturbing them during the breeding season.”

WTIP’s Kalli Hawkins spoke with Dave Grosshuesch, a wildlife biologist with the U.S. Forest Service based at the Tofte Ranger Station, as the owl irruption winds down, and to talk about ongoing owl research within the Superior National Forest. The audio from the interview is below.

Kalli Hawkins March 17, 2025