Count Down: Are Hawk Ridge’s Storied Goshawk Migration Booms Over?
Every 10 years for more than a century, thousands of Northern Goshawks migrate south from Canada through Minnesota. But in recent decades, the counts during irruption years have plummeted. And no one's sure why.
Perched high on a ridge overlooking Lake Superior, the four birders, arranged like arms on a weather vane, point their binoculars in every direction. With tally counters ready, their eyes and fingers move fast to record a phenomenon rarely witnessed in such relief against the sky: thousands of raptors and songbirds migrating south from the northern boreal forests of Canada.
Wings flapping, throats chirruping, nomadic blots in the blue. The watchers have their own code to demark the terrain where they spot a bird: in the clouds, through the trees, over the lake.
“What’s it look like going out over the summit, there?” calls Alex Lamoreaux, who is one of two real-time migration counters for the Hawk Ridge Bird Observatory in the northern Minnesota town of Duluth. The observatory has one of the longest-running hawk counts in the country.
Without skipping a beat, Lamoreaux realizes what he’s seen: “Whoa! Whoa! An Eastern Phoebe going below. Just dropped into the trees! Awesome.”
Duluth, with its hills and ridges hugging the southwestern-most tip of Lake Superior, is an ideal outpost to watch the migration. Many birds exiting the boreal forest choose to detour around the curve of the largest of the Great Lakes instead of flying clear across it. Hordes of birds pour down along the edge, their passage through Duluth marking the final bend before they disperse at the lake’s end.
It’s just past 7:30 a.m. on an early September morning. The pine needles are soft and the canopy below us is still green, signs that this year’s fall migration season is early. This morning at the hawk-watch overlook, a wooden platform pitched on a hill more than 500 feet above Lake Superior, Lamoreaux has already counted more than 250 warblers. It almost seems like there’s a bit of magic to it; identifying hundreds of birds each hour requires savant-like skills at spotting each bird’s shape, color, and flight pattern. Though the count lasts from dawn till dusk, this week Lamoreaux has yet to spot the ridge’s signature species: the Northern Goshawk.
Of the hundreds of thousands of birds that funnel around Lake Superior’s western shore on their journey south, the Northern Goshawk has become the mascot of Hawk Ridge. Within the United States, Duluth is still one of the best places to witness the hawks, which are notoriously difficult to study in part because of their secretive nature and their wide habitat range. The bird’s silhouette, the official logo of Hawk Ridge, appears on the observatory’s t-shirts and hats. The banding director, Frank Nicoletti, admits he originally took a job in Duluth just to see Northern Goshawks.
The hawks, larger than their relatives the Cooper’s and Sharp-shinned, are steely gray, almost blue, with light vermiculated stripes across their chests. Marked with bold white stripes over their eyes, they flap in slow, powerful arcs. In a normal year, Hawk Ridge typically counts fewer than 400 of the raptors—the numbers ebb and flow between 100 and 400 in any given year. But for more than a century, every 10 years or so, something wild happens. Thousands of Northern Goshawks descend south in exodus toward Duluth. That is, until recently.
Unlike many species, the Northern Goshawks don’t migrate en masse unless their winter prey base, mostly snowshoe hares and grouse, disappears. So when the hawks appear in Duluth, they come because they’re hungry. Yet something has begun to change. There was no Northern Goshawk irruption the last time it was predicted to happen. And the irruption before that was significantly smaller than the previous.
Why they no longer flee every 10 years or so is unclear. The bird is elusive, so it’s hard to know if the trend reveals a shrinking goshawk population or something about the birds’ ability to find food close to home. And studies on the birds' prey only complicate the question. It’s a mystery that some, like Nicoletti, can’t bear to give up hope on. Lamoreaux is more resigned.
“I knew there were truly goshawk irruptions,” Lamoreaux says. “But that’s just done for. It’s never going to happen again.”
Tracing the Irruptions
Northern Goshawk irruptions have been recorded since the early 20th century. In 1907, using evidence from taxidermied goshawks that were shot for mounting, American ornithologist Ruthven Deane noted in The Auk that there was an “unusual influx of these bold robbers of our game and looters of the poultry yard. I believe there has not been such a flight since the fall and winter of 1896-97.”
In 2002, an ornithologist writing about irruptions of Great Horned Owls found a remarkable correlation with Northern Goshawk irruptions. The researcher, named Ian Newton, was able to trace one goshawk event each decade going back to the 1880s. Accounts of these cyclical irruptions have since been recorded in detail at Hawk Ridge and other hawk watches, like the Cedar Grove Ornithological Research Station in Wisconsin.
In 1962, up on the ridge in Duluth, Hawk Ridge co-founder Jan Green remembers witnessing her first big flight of Northern Goshawks.
“They were flying in a way that I’ve not witnessed up there before,” says Green, who was 28 years old at the time. “That day I sat alone on Hawk Ridge and counted a steady stream of adult goshawks, in their striking blue-gray plumage, riding the updraft as they crested the ledges below me.” In all, she counted 169 Northern Goshawks in three hours.
Another big irruption followed a decade later in 1972, with 5,382 goshawks counted between September and November. In recent history, irruption years have been preceded or followed by a second year of massive sightings: In 1973, for example, 3,566 goshawks were counted at Hawk Ridge. In 1982, Hawk Ridge counters tallied 5,812, then counted nearly 2,000 the following year.
Over the years, Hawk Ridge’s counting methods have evolved from casual observation on autumn weekends, largely by volunteers, to 12-hour days, seven days a week during migration season by a combination of staff and volunteers. Even so, the irruption data has remained relatively consistent.
But in the 1990s, the peaks began a downward descent. Nicoletti, who was hired in 1991 as lead counter, helped tally 2,247 Northern Goshawks during the 1992 irruption. By 2001, the number was down to 1,170.
“Ninety-five percent of hawk watches in the country have not really seen a goshawk flight,” he says. “That’s what made me come here in ’91.” Unfortunately, he says, the 2001-2002 irruption was half of what he saw a decade before. “And we didn’t really have a 2011-2012 (irruption).”
The Disappearing Act
The secret to the disappearing goshawks might lie hundreds of miles north of Duluth, deep in the underbrush in the boreal forest of Canada. Here, fluffs of white snowshoe hares are known to rise and fall in dramatic ecological sync with the population of their primary predator: the lynx. As snowshoe hares breed and reach a peak, the lynx too will breed and then begin to disperse—and eventually die—when the hares become scarce. The peaks and falls are known to flow in 10-year intervals.
Researchers have recorded this phenomenon in numerous studies, and a few have examined this relationship in the context of Northern Goshawks. A 1994 study in Studies in Avian Biology found similar trends for the goshawks—in the years after hares declined, goshawk breeding success went down, too.
In Duluth, one researcher for years has been searching for clues within this ecological cycle. He wonders if it could help explain the disappearance of the irruptions. The operating theory is that when the hare populations decrease, the goshawks get hungry and head south.
Will They Return?
“If you see 5,000 [goshawks], you know there were a lot of birds and there are a lot of birds leaving,” says Dick Green, professor of mathematics and statistics at the University of Minnesota, Duluth, and of no relation to Jan Green. “The problem is, when you have low numbers, you don’t know if it’s because there aren’t very many birds or [because] they’re doing very well and they’re not leaving.”
Green has collected historical data about Northern Goshawk irruptions and analyzed statistics from various hawk count sites. He’s also pored over research on snowshoe hares, including recent studies that indicate other alarming trends: the size of the snowshoe hare peaks are declining and the synchronicity of the predator-prey populations across Canada is no longer consistent.
Rudy Boonstra, a co-author on several recent snowshoe hare population studies and a professor of ecology and evolution at the University of Toronto, says that remarkably, lynx populations have stayed much higher than usual during the last snowshoe hare low, which ranged from 2010 to 2014.
“What’s keeping them in the system when there are no hares?” Boonstra says. “Obviously they’re not living on air, so presumably they’re killing other things. The lynx changes are presumably a proxy for what’s going on in the goshawks as well.”
Boonstra says increasing temperatures in northern North America could also be to blame. The impacts of a warming planet on boreal forests are catalogued extensively in scientific literature and could include drying soils, more wildfires, and more vegetation, among other things. These changes could have “cascading effects,” he says, including better survival rates among both lynx and hares during winter. But “exactly how these things translate into numbers is unknown,” he says.
There are no easy answers. Some speculate logging in the boreal forest could be destroying Northern Goshawk habitats, though studies have cast doubt on this theory. For Green, the question remains: Is there going to be an irruption next year, or the year after that?
A New Focus
Hawk Ridge count director Karl Bardon often goes out at night to listen and watch for nocturnally migrating birds. On nights with a full moon, he’ll watch birds passing across the light.
One August evening in 2013, Bardon saw a “continuous wall of [Common] Nighthawks came through just before sunset.”
The vanishing spectacle of the goshawk irruption has, in some sense, been succeeded by an effort to count thousands of other birds, like the Common Nighthawk, and several species of non-raptors, in real-time.
Each morning at sunrise, Bardon stakes out near the Lake Superior shore while Lamoreaux takes counts from the ridge. On Lamoreaux’s first day of work this past year, the pair recorded more than 91,000 non-raptors.
Bardon says the effort to count non-raptors is important. Although many hawk count sites record non-raptors, few aside from Hawk Ridge and Cape May Bird Observatory in New Jersey do so systematically. High-quality binoculars and telescopes have made identifying and counting these birds much easier than what was possible in earlier years.
The data is groundbreaking, Lamoreaux says, because it gives a snapshot of all the species migrating from the boreal forest.
“Because now we’re 30 years removed from the goshawk irruptions, I think we should take a new direction where we focus on the songbird migration or the Common Nighthawk migration,” he says. “Now we’re evolving to take on these new phenomena.”
Accounting for hawks and non-raptors, the daily tally at Hawk Ridge can be enormous. That early September day turned out to be one of the busiest flights of the season: Lamoreaux and the others counted nearly 12,000 migrating non-raptors and over 9,400 hawks. By day’s end, the watchers spotted two goshawks, the first they’d seen all week. By the end of October, typically the busiest month for goshawk sightings, the number totaled 130. It's not the smallest amount Hawk Ridge has had during a non-irruption year, and yet it tells us little about what is happening with the population—much less whether Hawk Ridge's days of goshawk irruptions truly are over.