For a quarter-century, Peter Sherrington and a few thousand fellow eagle enthusiasts have been scanning the skies west of Calgary, counting raptors.
It all started by accident 25 years ago to this day.
On March 20, 1992, Sherrington was out with a friend in the Mount Lorette area of Kananaskis Country when they spotted a golden eagle around 11 a.m., which they found interesting because they believed it to be relatively rare.
They then saw two more while they were having lunch. Then another.
By the end of the day, they counted more than 100 golden eagles, all heading northwest.
"I thought, this is not random," Sherrington told the Calgary Eyeopener on the 25th anniversary of that day.
"This is something that is worth studying."
He went back out a couple days later with a larger group and together they counted 250 eagles in a single afternoon.
They had discovered a golden eagle migration route — more like an expressway, as it turns out — that persists to this day.
"The following fall, I went out and had over 2,000 golden eagles coming the other way," Sherrington said.
"And that in itself was interesting because in most parts of the world, raptors use one route in the spring and a different route in the fall. But this was one-stop shopping."
The next spring he counted 4,200 golden eagles, including 850 in a single day.
Citizen science project
The discovery led to the creation of the Rocky Mountain Eagle Research Foundation and its regular raptor count has become one of the largest and longest-running citizen science projects in the world.
Sherrington is back out tallying eagles this spring along with anyone who cares to join.
"Just come along," he said. "Dress warmly. Bring some binoculars. You don't even need to do that; we have spare binoculars and telescopes."
The eagles follow a route along the front ranges of the Rockies from Waterton National Park to northeastern British Columbia. Some have been known to fly that span in a southerly direction in less than two days.
"The speed that these birds can glide is quite phenomenal," Sherrington said.
"They're not exactly flocks, but they're all going on the same routes."
The birds are actually quite "antagonistic" to one another but often end up bunching up in a situation Sherrington likens to heavy traffic hitting red lights along Calgary's Macleod Trail in rush hour.
"The traffic lights, in this case, are generally weather systems," he said.
"So they get held up for a couple of days by snow and low cloud and then it clears — which is the green light — and there's a sudden flood coming through. We've had up to 200 golden eagles in an hour coming through under those conditions."
He believes the route to be an ancient one that eagles have followed for 11,000 years.
The group's 2017 spring migration count is underway now and continues until April 22.
Observers will be at the Mount Lorette site daily and you can find more information about how to join them here.
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With files from The Calgary Eyeopener.