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Ghost Chasers
It happens only every so many years, when the days grow cold and dark. Suddenly, birders’ phones start to ring. The message: The owls are here!


Known for their secluded boreal haunts and their ability to slip silently through stands of tamarack, spruce, and aspen on wings that span nearly five feet, great gray owls have been aptly called the phantoms of the northern forest. Like many ghosts, they are notoriously hard to track down, even by dedicated seekers who are willing to venture into the boggy, mosquito- and black fly-ridden places that are the owls’ preferred nesting sites throughout most of their range. But now and then, in the dead of winter, they materialize in unexpected locations far from their usual breeding range, thrilling keen birders and casual observers alike.

When great gray owls venture south of their boreal breeding grounds, which extend across the northern half of the continent from Alaska to western Quebec, they typically do so in large numbers. Known as irruptions or invasions, these mass movements occur roughly every three to four years, sometimes spreading from the Great Lakes to the Atlantic, at other times concentrated in only a few states or provinces. Boreal owls and northern hawk owls make similar movements, but neither are as conspicuous as their larger relatives. Whether widespread or localized, the sudden appearance of numerous massive, stately great gray owls transforms the winter landscape.

North America’s most recent major great gray owl irruption, in 2004–2005, was hailed as the largest ever, garnering attention from The New York Times and CNN and attracting owl lovers from all over. That winter a few of the birds strayed as far east as Maine and as far south as central Iowa. However, the greatest spectacle was in Minnesota, where more than 10,000 great grays were reported. An amazing 256 of those sightings were logged by two industrious birders in a single day.

Analysis of the Minnesota records indicates the owls represented at least 5,225 individuals—13 times the previous state record, set in 2000–01. “I don’t think the 5,225 figure is out of line,” says Dave Grosshuesch, a U.S. Forest Service biologist who helps out with the owl monitoring work for the Hawk Ridge Bird Observatory near Duluth. “It’s probably low, to be honest.”

He speaks from experience. All winter long Grosshuesch ventured out several mornings a week, equipped with banding gear, a fish net attached to an extension pole, and live mice to lure hungry owls, of which there was never a shortage. “I’d go back on separate days to a spot where I’d caught a great gray owl, and I’d catch a different one each time.” Altogether, he banded 305 during the irruption.

In Minnesota and elsewhere, the winter tally also included an unfortunately high number of owls—800, to be exact—that were struck by vehicles, a consequence of their inclination to hunt along roadsides and their lack of experience with traffic, an uncommon hazard in their boreal home. The majority of reports about live owls came from binocular-toting observers like Arlys and Curt Caslavka of Middleton, Wisconsin. These longtime members of the Madison Audubon chapter first learned of the invasion in early January, when a friend emailed them to share the news.

“We’re not people who hear about a bird and just take off to go see it,” says Arlys, a retired primary-school teacher. But that’s exactly what they did this time. In more than 30 years of birding, neither of the Caslavkas had ever seen a great gray owl, so the next day they were on their way toward northern Minnesota.

Nearly 300 miles later, not far from the town of Superior, the couple got their reward. Perched in a roadside tree was the unmistakable form of North America’s tallest owl, standing nearly two feet, its bulky body patterned in soft shades of gray and brown, a neat, white “bow tie” at its neck, and small yellow eyes peering out of a heavily ringed facial disk. “It’s such a beautiful face,” says Arlys, “so gentle and wise-looking.”

As exciting as that first sighting was, it was only the beginning. Arlys still speaks with awe about the 69 great grays she and Curt encountered during their drive farther north to Eveleth, Minnesota, and the Sax-Zim Important Bird Area, a location about 50 miles north of Duluth that Mark Martell, director of bird conservation for Audubon Minnesota, says was the epicenter of the irruption and remains a reliable place to see the owls nearly every year. During their trip, the Caslavkas saw great grays singly in some areas and clustered by the score in others: plunging for prey in snow-filled ditches along rural roads; perching on rooftops in Superior; roosting on driftwood logs and on the beach at Wisconsin Point. “Seeing the owls was a privilege,” Arlys says. “One of those experiences [that] just fill my heart up.”


While the 2004-05 invasion is undoubtedly the best-documented instance of a great gray owl irruption, the written record of this phenomenon dates back to the early 19th century. In 1831 John James Audubon got word of a “great cinereous owl”—as the species was then known—that had made its way to the coastal Massachusetts town of Marblehead, near Boston. Found perched on a wood pile one February morning, the sojourner had been taken into custody by a Mr. Ives, who kept it alive for several months on a diet of fish and small birds. To Audubon’s great disappointment, the owl died before he could get there to see it, and not a single bone or feather remained for him to examine.

Although the Marblehead owl was reportedly “very fond” of the food it received from Mr. Ives, great grays normally prey on small rodents, with a decided preference for voles. In fact, this dietary specialization is likely the main impetus for their periodic mass movements. Canadian James Duncan, manager of biodiversity conservation for the province of Manitoba’s Wildlife and Ecosystem Protection Branch and one of the leading experts on this species, believes great gray owl irruptions are a direct result of their “boom-bust economy.”

“The real cause of these large-scale movements of large numbers of birds,” he says, “is a couple of great years of reproduction—and that means they’ve had lots to eat and they’re breeding fairly well—followed by a precipitous crash in the prey population.”

Now in his 26th year of studying great gray owls in southeastern Manitoba and neighboring parts of Ontario and Minnesota, Duncan has banded more than 2,000 great grays and radio-tracked 103 of them, some for six consecutive years. He has also investigated their prey relationships by dissecting thousands of regurgitated owl pellets, trapping countless rodents, and reading his way through a four-foot-high stack of literature on small mammal population cycles that sits on his office floor.

One of his findings is that meadow voles comprise more than 90 percent of the great gray owl’s diet in this region—despite being less abundant than some other small mammals, such as red-backed voles. Even when meadow voles become scarce, as they do about every three years, the owls seem disinclined to try other fare. Boreal-based great grays “don’t appear to be adept or adaptable to taking other things,” Duncan says. “If the meadow voles crash, the owls seem to be doomed.” Their only option is to go in search of a place where voles are more plentiful.

Duncan’s radio-tracking research clearly shows that how owls respond to a dearth of prey determines their fate. “The ones that moved, survived,” he says. “We’d find them alive and happy up to 500 miles away the following summer. And the birds that failed to relocate, perished. They either starved to death or were killed by predators.”

This close link between great gray owl movements and prey populations appears to apply throughout the boreal region, but not all members of this species are at the mercy of the volatile meadow vole market. Those that inhabit coniferous forests in the northern Rockies and other western mountains are perpetual homebodies—a lifestyle made possible by the availability of a stable alternative food source, namely the pocket gopher.

“The scenario in places like Northern California, Wyoming, and Oregon,” says Duncan, “is that when the meadow vole populations are high, the owls do quite well and they nest, [although] they don’t raise as many young as they appear to raise in the boreal forests. Then the following year, when the meadow vole population crashes, they seem to be able to sustain themselves on pocket gophers. They may forgo breeding for a year or two, but they’ll maintain their home range and then nest in the same nest or one close by during the next vole increase.” 

The contrast between these two survival strategies is most dramatic in terms of the area needed to support an individual great gray owl over its lifetime. “Some of the largest movements of radio-tracked birds in the montane situations are 19 miles, their whole life,” says Duncan. Boreal dwellers, on the other hand, regularly move between breeding sites up to 500 miles apart. A bird born in northern Minnesota, for example, might first breed near Hudson Bay, then return two or three years later to nest in the same stand of trees where it was raised or in one nearby.

A boreal great gray owl’s lifetime home range “is about as big as that of an African elephant,” says Duncan, and this has important implications for the species’ long-term prospects. The greatest threat to these owls is habitat loss, primarily resulting from logging, which is rapidly converting boreal ecosystems into continuous monocultures of trees of identical age and species. “They really depend on huge areas to live out their lives, so we need to think about their lifetime habitat requirements and maintaining at least a portion of the boreal forest in a suitable state,” Duncan explains. “Large areas with all the same kinds of trees and no openings just won’t work for them.” What they need is a mosaic of wet meadows and mature forest, providing productive vole habitat adjacent to stands with good nesting potential.


Although great gray owls make the news when they invade south and east of their breeding range, Duncan has found that hunger can also propel them in the opposite direction. All of his radio-tagged owls ended up relocating north or northwest of the study area when local vole populations bottomed out, even if they initially ventured a short distance southward. “Northern irruptions go relatively undetected, at least by science,” he says, partly because there are few birders reporting from remote areas of Canada and Alaska.

One initiative that may help close this knowledge gap is eBird, an online checklist program launched in 2002 by the Cornell Lab of Ornithology and National Audubon. “It’s a great tool for tracking a population as it shifts over the course of a winter or an irruption event,” says Brian Sullivan, one of three project leaders with Cornell. Although eBird was still in its infancy in 2004–05, participants filed enough great gray owl reports from around the Great Lakes to generate meaningful maps of the invasion in that region. For Sullivan, “It was pretty neat to be able to visualize for the first time on a regular basis what was going on.”

Being able to plot the birding community’s response to the invaders was similarly satisfying. “We definitely saw a huge spike in the number of observations coming in from areas where we otherwise would have very little participation,” he says. “A species like the great gray, being as cool as it is, really draws a lot of folks out to look at it.”

Sullivan himself traveled several times over the winter from Ithaca, New York, to Enterprise, Ontario, 215 miles away, one of the invasion hot spots, just to see the owls. When he arrived, he found dozens of the celebrity birds perched obligingly on roadside fence posts, as well as carloads of owl watchers, both tourists and locals, admiring them.

No one knows when the great grays will again head south in high numbers—it could be this winter, or not for another decade—but when they do, their human fans will undoubtedly flock to greet them. Meanwhile, the owls will carry on their reclusive lives in the boreal forest and biologists like Duncan will continue trying to unravel their secrets.

Freelance writer Frances Backhouse is the author of Owls of North America and Woodpeckers of North America.

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State of the Bird:
Great gray owl
(Strix nebulosa)

Looks: Huge owl (although much of its apparent two- to three-pound bulk is feathers), standing nearly two feet tall. Finely barred and mottled brownish-gray, with two noticeable white marks on neck.

Behavior: Elusive, hiding inside forest by day, hunting in open areas at dusk or at night. However, the great gray is known to hunt by day when feeding nestlings and may also hunt in daylight when food is scarce. Despite its large size, it feeds mostly on voles or pocket gophers.

Range and habitat: Thinly distributed across boreal zones of Canada, Alaska, and northern Eurasia; local farther south in high mountains, to Wyoming and California. In boreal areas favors dense forest broken by wet meadows, bogs, and clearcuts for foraging. Occasional winter invasions bring some into northeastern United States.

Status: Because of its remote wilderness range and irregular winter distribution, we have no good way of tracking its population trends.

Threats: Logging, energy development, peat extraction, and clearing for agriculture in the boreal forest pose a risk to this owl, and to many other species.

Outlook: Probably secure if substantial blocks of boreal forest can be protected.—Kenn Kaufman

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