As I was taking in the view of snow-laden trees, massive peaks, and Kluane Lake—the largest lake in the Yukon—I spotted the silhouette of a bird perched high in a bare tree. The northern hawk owl, a rare sight in the wild, pinned its yellow eyes on me and let out a sibilant screech that nearly made the hair stand up on the back of my neck. That episode is now long past, and I have since spent years studying owls of the northern forests. But I often think back to that moment as the beginning, when I first took notice of one of the least studied birds in North America, and gained a direction for my work.

Later on, as I read the literature on hawk owls, I found that most of the information in textbooks and field guides originated in Scandinavia, where the species has long been studied. But I wondered: did it make sense to assume that hawk owls in the New World have the same lifestyle as their cousins in the Old? Aside from some anecdotal reports, few nests in North America had been described. The most extensive work had been done in the mid-1980s by Kenneth Kertell, now a senior scientist at SWCA Environmental Consultants in Tucson, Arizona. Kertell had studied hawk owl behavior at six nest sites in Alaska’s Denali National Park.

My colleagues and I carried out field investigations for the Kluane Boreal Forest Ecosystem Project from 1987 through 1993. In the course of our work, we were able to expand the story of New World hawk owls. We now know that these owls diverge from their Eurasian counterparts not only in aspects of their breeding biology, but also in their behavior, which reflects certain basic differences in the ecology of their boreal homes.

Northern hawk owls are unlike most owls, and, as the name "hawk" suggests, act in some ways like diurnal birds of prey. They hunt in broad daylight and rely on their long tails—shaped more like falcons’ tails than owls’—to maneuver in rapid flight. Hawk owls lack the comblike structures along the outer edge of their primary feathers that give most owls silent flight. Their sense of hearing is only so-so for a bird of prey; the keen ability to perceive sound that enables other owls to pounce accurately on prey in the dark is absent in hawk owls.

They are, however, truly hawk-eyed. In a tag-and-release program in Alberta, Canada, field ornithologists "reel in" owls, using a fake mouse attached to a fishing line. Hawk owls are routinely attracted to the small lure from nearly a mile away.

The "northern" in the owls’ name denotes their range.Hawk owls live and nest in the taiga, the subarctic band of forests that circle the northern reaches of the globe from Alaska to eastern Canada and from Scandinavia to Siberia. They are nomadic, readily shifting to new nesting areas depending on the relative abundance of their small mammalian prey. And in winter, they sometimes irrupt, or move south of their more usual range in large numbers. The North American subspecies, Surnia ulula caparoch, makes occasional winter appearances on farmland as far south as southern Canada and the northernmost regions of the contiguous United States.

Shortly after my arrival at Kluane I again spotted the bird that had caught my eye on that first day. I watched as it made its screeching call, and then I heard a second, responding hiss. When I raised my binoculars to a snag, or jagged top, of a broken, burned-out tree, I found myself locked in a gaze with a female on her nest.

After that I began spending as many as twenty hours a day in continuous watch and was rewarded with intimate glimpses of the pair’s family life. I was transfixed: One day a grizzly walked by about sixty feet away, but we ignored each other. Bands of mosquitoes buzzed on the netting around my face. A yellow-rumped warbler grew so accustomed to me that he started to peck insects from my wool pants. I became part of the forest.

The male owl hunted fairly close to the nest. Every few hours he returned to a low perch with a dead vole or mouse, called out, then decapitated the prey before presenting the remains to the female. He usually dismembered heavier prey at the site of the kill, then delivered morsels directly to the nest. Occasionally he tucked food remnants into tree holes and crannies for safekeeping.

Bands of mosquitoes buzzed on the netting around my face. A yellow-rumped warbler grew so accustomed to me that he started to peck insects from my wool pants.

When I reached into one hiding place where the male had cached some meat, I grabbed the hind part of a juvenile hare.

Hawk owl eggs, like the two in this nest, hatch after about thirty days of incubation. The parents cater to the owlets in the nest for another three or four weeks. When the owlets fledge, they exercise their new ability to fly by leaving the immediate nest area, but the parents continue to supply them with food for a few more weeks.

In Scandinavia and Russia, hawk owls (subspecies Surnia ulula ulula) are classic specialists: in the breeding season 95 percent or more of their diet is made up of small rodents such as mice, lemmings, and in particular, voles. In Denali National Park, Kertell had discovered that voles made up only 70 percent of their diet; young snowshoe hares and squirrels made up the balance. He suspected that the population of hares influenced the Denali owls’ diet. Having held in my hand the bloody evidence that the nesting birds in Kluane were also feeding on hares, I was eager to find out more about the link between hawk owls and hares.

Fortunately, my enthusiasm for hawk owls spread to my colleagues in the long-term ecosystem project. Some of them took censuses of the prey animals in the region, and their data on how the densities of voles, hares, lemmings, and squirrels varied from year to year proved to be the key to part of the hawk owl story. With the help of some volunteers, we found nine nests, the most ever included in a single hawk owl study. We systematically recorded all our observations of hawk owls, and collected and dissected their pellets (small bundles of the regurgitated hair and bones of prey).

The boom and bust cycle of the populations of small mammals is a phenomenon of the taiga and tundra regions of the north. For some still undetermined reason, the numbers of voles, lemmings, and hares soar in some years and plummet in others. Snowshoe hares peak about every ten years. When they were most numerous, so many hares would be hopping along and across the Alaska Highway that I would have to slow my car way down, to avoid a mass slaughter. In contrast, at the low end of a hare cycle, I could walk in the woods for hours and see hardly any hares.

But not all boreal-forest ecosystems are the same: in Scandinavia the populations of small rodents, including various species of voles and lemmings, peak together roughly every three to four years. The biomass, or total organic weight, of those mammals is substantial. And as that biomass cycles between boom and bust, it accounts for more population change throughout the ecosystem than do the biomass cycles of any other vertebrate animals in the system.

In North America, however, the population cycles of prey animals are different. At the peak of their ten-year cycle, snowshoe hares, in terms of biomass, "outweigh" all other vertebrate species. Vole populations also peak and crash in North America, but do so more irregularly than those of hares.

In the Yukon, as in Eurasia, hawk owls seem to prefer voles to other prey. Our statistical analysis showed that they fed on voles significantly more often than they would have if their dietary mix had matched the densities of the available prey. But the diets of Yukon hawk owls are buffered by the availability of other prey. When we began observing nesting hawk owls in Kluane, both vole and hare populations were rapidly increasing. Later, when the vole population crashed but the hare population continued to grow, the hawk owls remained in the nesting area. Voles did drop to about 30 percent of the hawk owl diet by biomass, and young hares and squirrels rose to about 50 percent. In a peak year for hares, the hares edged out the voles as a source of meat for both adult owls and their owlets. But the shift in the owls’ diets from voles to hares didn’t hurt breeding.

In Scandinavia, hawk owls usually breed only during bursts in the population of voles and lemmings. The average nest holds six or seven young, but at times even larger broods seem to be common. (The record is thirteen, but how many young in that megabrood survived is unknown.) Although investigators have not located as many nests in North America as they have

Hawk owls were condidered vole specialists, but when I reached into one hiding place where the male had cached some meat, I grabbed the hindquarters of a juvenile hare.

in Scandinavia, hawk owls in the Yukon consistently rear smaller broods than do European hawk owls, between three and five nestlings.

No matter where hawk owls raise their young, breeding coincides with the availability of prey. Owlets fledge in late May or June, just when the boreal forest is teeming with inexperienced young prey animals. At Kluane the hares brought to the nests by parent hawk owls were, on average, only twenty-two-days old.

Hawk owls are also thought to scavenge the remains of adult snowshoe hares in winter. But we discovered, to our surprise, that these owls, weighing less than a pound, also attacked and killed live adult hares four times their body weight. The hares might have been weakened individuals; even so, the performance speaks to the ferocity and daring of North American hawk owls. The hawk owls of Alaska and Canada are about 6 percent larger than Scandinavian birds. Perhaps their size is an adaptation that increases their success in capturing larger prey.

It seems odd that the hawk owl should emerge as a vole specialist at all, given the owl’s strong physical resemblance to birds that prey on other birds. Similarly shaped raptors, such as peregrine falcons and goshawks, are adept at the agile pursuit and rapid capture of birds in flight. But hawk owls, perhaps descended from bird hunters, are skilled aerial predators in their own right. In Scandinavia, during harsh winters when voles are scarce or inaccessible under thick snow, most of a hawk owl’s diet can be made up of birds. During winters, our research team in Kluane saw hawk owls kill spruce grouse roughly as big as the owls themselves.

The ability to take advantage of a range of prey animals may give all hawk owls the flexibility they need to survive in harsh and variable northern climates. Compared with the Scandinavian birds, North American hawk owls turn out to be a bit larger, to have fewer young per clutch, and to be less specialized. As investigators gain a better understanding of the species across the entire northern forest, it’s becoming clear that this small, fierce, and versatile owl may have been underestimated.

Recently, Wayne Lynch, a Canadian wildlife photographer, and his field assistant Julia Burger witnessed an example of hawk owl predation that had never previously been reported. They were walking to a hawk owl nest near Chip Lake, Alberta, when they flushed an American wigeon. One of the hawk owl pair, perched in a snag above the nest, immediately attacked, hitting the duck in flight and riding it to the ground. Opportunity, even for an unusual meal of waterfowl, had quickly brought out the aerial hunter. The hawk owl, its mate, and their young feasted on duck for days.

Christoph Rohner is a zoologist and writer who lives in Edmonton, Alberta, Canada.

Copyright © Natural History Magazine, Inc., 2008