Researchers on the frigid shores of Hudson Bay are changing our picture of the prince of birds
By Gordon Court | Published: 7/22/2008
Bundled up in the latest expedition-standard cold-weather gear, I am lying on my back along the seat of a snowmobile, parked on the sea ice on the west coast of Hudson Bay, 260 miles south of the Arctic Circle. Temporarily forgetting my respect for foraging polar bears, I have sought this repose to watch the sky.
I rest the full weight of my binoculars on my cheekbones and steady them with my hands to keep them fixed on a miniscule silhouette of a bird circling several hundred feet above me. It is no ordinary bird — it is a member of a species that has been described as the prince of birds, a feathered rocket known as the Peregrine Falcon.
A female of the subspecies tundrius, the falcon has only recently arrived on her territory near Rankin Inlet, in Canada’s Nunavut Territory, after completing a migration of thousands of miles from her wintering haunts in South America. I can only wonder about her travels during the eight months she was away from the breeding grounds.
As I follow the drifting speck of a bird, I become more interested in her behavior. I know she is hunting. Years before, I became addicted to watching Peregrines prey on gulls and terns over rivers that drain Canada’s boreal forest. The hunts were usually classics, incorporating the widely reported dive, or stoop, from high altitudes; once you witness a stooping Peregrine, you will always long for an encore. The bird in my view shows all the precursors of a real air show — a chase is imminent.
Positioning my binoculars is difficult — my speck is wobbling in the eyepieces. But then, as arm fatigue hits maximum, she drops — not a perpendicular stoop but still incredibly fast. In an instant, she changes her flight from vertical to horizontal and streaks over patches of snow and tundra at a blistering rate, perhaps five feet above the ground. I anticipate seeing her quarry at any second. Will it be a Rock Ptarmigan or a Snow Bunting? Both species are flashy and white and easy to see.
A dip to the ground – nothing flushed – nothing caught. No, wait! Something’s in her talons. She’s flying back to her nest high on a cliff. I grab my spotting scope and focus it to look into her huge ebony eyes, each sharply framed by vivid yellow lids. The contrast of her gray and white plumage is striking. But what of the prey? As she plucks it, the mystery is solved. The most celebrated bird hawk of medieval falconry is devouring a lowly rodent — a brown lemming.
The hunt took place back in 1982. At that time, I understood that the Peregrine Falcon specialized on avian prey. Mammals, including rodents and bats, were sometimes recorded in its diet but usually made up only a miniscule portion. Now, after 26 years of research on the tundra and sea ice at Rankin Inlet, I know that rodents can account for nearly one-third of a tundra Peregrine’s food — more than any other population yet studied. Moreover, the use of wingless appetizers introduces a uniquely dynamic element to the bird’s breeding success.
Located along the 63rd parallel, about 300 miles north of Churchill, Manitoba, Rankin Inlet is an outpocket of the inland sea, Hudson Bay, and a forbidding place.
The climate is far more severe than the latitude would suggest — snowmobile travel on the sea ice is possible until late June. Yet it is home to one of the densest and most productive breeding populations of the Peregrine Falcon anywhere in the world. When a colleague and I began studying the birds in the early 1980s, we focused on an area of about 280 square miles, about half of which was the water and ice of the inlet. The landmass enclosed within our arbitrary boundaries is a largely flat expanse of tundra and rock interspersed with a few small cliffs, few of them over 75 feet in height, seemingly beneath the notice of a dynamic cliff-nester like the Peregrine.
Compared with most rocky heights where Peregrines breed, the inlet looks to have about as much suitable nesting habitat as a parking lot. But in this relatively tiny area, up to 28 pairs have established territories — an average of about one pair in every 10 square miles. Incredibly, from a single windswept vantage point near the Inuit hamlet of Rankin Inlet, I can see 11 active aeries, all of them within a brief boat ride of one another.
My earliest work was a simple investigation of when and where the falcons nested and what they ate, but I quickly became enthralled with their territoriality. After arriving in mid-May, most will pair and lay eggs by the first week in June. Unlike non-migratory populations in which adults occupy territories year-round, the birds select mates and establish territories in a short time. And nests can be located as little as 600 yards apart, so opportunities for conflict abound. Tremendous aerial battles over mates and cliff sites are common.
I found that females were interested mainly in short-distance defense of the nest. They would usually chase only other females that ventured within a few hundred yards of their cliff. Males, on the other hand, were the true long-range interceptors. It is their behavior that may ultimately enforce inter-nest distances and explain the pattern of nest-site placement over the landscape.
Apparently, territorial males can identify potential competitors at a great distance. Territory holders attacked intruding males over a mile from their cliffs. Often, when the defenders overtook the interlopers, the two entered into vicious physical combat. In several instances, we saw birds struck outright by stooping defenders. On other occasions, falcons locked talons and fell several hundred feet to the ground.
The fall migration routes of four satellite-tracked Peregrine Falcons are shown in orange, green, purple, and blue. Each dot represents where the bird was located during its journey. The short blue line plots the route of a falcon on its way north the following spring. Its transmitter stopped working.
Data courtesy the Government of Nunavut
In 1994, scientists placed satellite backpacks on four Peregrine Falcons at Rankin Inlet. The data transmitted from the backpacks allowed researchers to track the birds as they migrated to their wintering range in South America.
One transmitter (green) stopped sending signals over the Gulf of Mexico, suggesting the bird died, but the other three flew to southern Brazil, covering an average of 8,900 miles.
In addition, more than 900 falcons have been captured and marked with leg bands. More than two dozen of them have been found — in southern Texas, near Chesapeake Bay, and as far away as Peru, Uruguay, and Argentina.
“Through this and other studies, we have learned that Peregrines spend little time wandering between breeding areas and wintering grounds, sometimes flying from the Arctic to South America in less than 10 days,” the researchers wrote. “While on the wintering grounds, they usually stay within a good feeding area for the entire winter, returning very quickly to breeding areas in the spring.”
The scientists plan to place solar-powered transmitters on at least eight adult males this spring.
Such violence sometimes resulted in injuries or death to combatants. Inuit hunters traveling the land in the early spring brought us birds with shattered wing bones, dislocated shoulder joints, and other injuries produced by horrific collisions. On several occasions, we found dead adults at the base of nest cliffs, usually uneaten but with their heads bitten off. Even if a physical battle did not materialize, the territorial challenges were invariably spectacular. Males returning to cliffs combined exaggerated wing-pumping display flights with daredevil stoops terminating within a few feet of perched females.
Hand in hand
I was fascinated by the spectacular fights, recognizing that the displays are a primary mechanism for spacing avian territories. I also knew from the ornithological literature that food supply and the degree of territorial behavior go hand in hand. When food is abundant, birds are less territorial, territories are smaller, and a greater number of individuals share the landscape. The opposite is true when food is scarce. For example, the size of shorebirds’ feeding territories varies with the richness of the invertebrate food resources in the mud underfoot. And nectar-dependent hummingbirds vigorously defend their foraging areas, the size of which contract and expand daily depending on the number of open blossoms in the flower grove.
For jaegers and owls, which prey on lemmings, territory size and the number of breeding pairs tend to wax and wane annually depending on the number of rodents. Conversely, for other species of birds with less specialized diets and stable food resources, like Peregrine Falcons, territory size and the number of breeding pairs are remarkably constant.
By utilizing a wide range of prey, they don’t suffer the vagaries of boom-or-bust food supplies. The size of their breeding populations and reproductive output rarely change much from year to year in any given location.
But what, I wondered, might happen to Peregrine numbers and breeding success when rodent populations peaked? What would we see in years when the tundra was overrun with lemmings? Would the falcons capitalize on the bounty? If so, would they continue to regulate their numbers like most of the populations studied the world over?
Or would superabundant and easily captured mammals turn them into high-speed mousers with flexible territory sizes and variable reproductive output? We waited anxiously for the next super-lemming year.
Lemming numbers are known to peak roughly every four years. The phenomenon has been studied well, and explaining the mechanism behind the cycles has been the lifelong pursuit of ecologists like Charles Krebs from the University of British Columbia. Not only are lemmings much more abundant at peak, he and his colleagues have found, but their average body size is also much larger than in non-peak years. This, perhaps, is the gravy that pushes true lemming predators like hawks and owls into high reproductive gear during a fat year.
Bounty on the tundra
During most of the short arctic summers of the last two decades, the Peregrines in our study showed little sign of benefiting from such fat, displaying a predictably consistent population size and reproductive output. In most years, roughly the same number of pairs initiated breeding, and the number of young fledged was within 20 percent of the long-term mean.
In high-lemming years, however, everything changed. In the spring of 1985, for example, signs were obvious that we were in for the lemming spike of the decade. During a helicopter survey, we noted that the population of nesting Rough-legged Hawks had exploded to 40 pairs, up from 3 the previous year. We were even more amazed to discover totally new adult Peregrine pairs at alternative nest sites of established pairs, others on previously unused cliffs, and birds nesting between known territories on poor-looking habitat. One pair even nested on a boulder in the middle of a stream, not six feet off the ground. Overall, the number of adult pairs on territory increased in one season by about 30 percent, and the number attempting to breed increased by more than 50 percent.
From an ecological perspective, the falcons demonstrated a classic numerical response to increased prey abundance. (Recognized lemming predators such as Snowy Owls and Rough-legged Hawks did the same.) Established Peregrine pairs allowed the new pairs to squeeze between their territories, I suspect, because the territory holders were well fed and therefore less vigilant.
Surplus of adults
The appearance of new pairs in itself was fascinating evidence of a non-breeding surplus of adult falcons. Among raptor population biologists, the so-called “invisible floating population” is a subject of much speculation. At Rankin Inlet, Ph.D. student Robin Johnstone from the University of Saskatchewan tested for the presence of floaters by temporarily removing Peregrines from their territories in the early spring. In about 80 percent of territories, birds not known to the study population replaced territory holders within 24 hours. One male was replaced in less than four hours.
In addition, we documented a massive change in reproductive output. Adults reared more than double the average number of chicks from the previous four years. Unprecedented for the species, the profound, short-term increase occurred not because established pairs had extra broods but because more pairs initiated breeding. Our observation was consistent with previous research that found that the proportion of raptors attempting to breed in a season is influenced directly by food supply during the pre-laying phase of the nesting cycle.
The extensive use of mammals by Peregrines has been documented only at Rankin Inlet, although it may be common for falcons nesting throughout most of the coastal and interior barrenlands of Nunavut. Intriguing evidence also suggests that an equivalent situation may exist on the Yamal Peninsula in northern Russia.
It is certainly easy to see how a tradition of mammal feeding could have evolved. During the summer, a large portion of a falcon’s diet consists of young passerines and shorebirds, most of which are still flightless and therefore easily plucked off the tundra. A hunting Peregrine probably sees little difference between a lemming running between cover and a wobbly fledgling hopping from its nest.
The ability to feed on just about any small animal is perhaps the primary reason the prince of birds has colonized almost every corner of the globe.
Gordon Court is a wildlife biologist for the Alberta Fish and Wildlife Division and an award-winning photographer. He has worked on the recovery of the Peregrine Falcon in western Canada for three decades and has served as Canadian director of the U.S.-based Raptor Research Foundation.