Winter grays

Natural History, Feb, 1998 by Gordon Court

Mid-October, 1996: A barley farmer near the community of Newbrook in Alberta, Canada, surveys the sodden remnants of his precious crop, kicking at the rotting swaths with disgust. Like most grain growers here on the southern edge of the boreal forest in this part of the province, he knows that the wet autumn has sealed the fate of this year's harvest; most of it will be left on the ground. The energy-rich seed heads will soon be devoured by geese and ducks as they gather here before their journey south; then mice and voles will move into the furrows and also reap a barley bonanza. In early winter, under the protective blanket of snow, the fattened rodents will begin to breed in prodigious numbers and in turn attract mammal-hungry predators from miles around. This year the abundance of rodents heralds a natural history phenomenon: an invasion of great gray owls.

My colleagues Ray Cromie and Trevor Roper - owl banders rather than farmers - monitored the crop conditions in east-central Alberta in the fall of 1996 with enthusiasm. As the scent of moldy grain filled the air, they could almost smell the owls coming. On November 11, during a heavy snowfall, the great grays came in like a tidal wave. Ray phoned to alert me when the first birds were spotted just north and east of Edmonton, Alberta, where I live. Despite the storm, I bundled my family into the car and headed for the town of Opal and into what I can only describe as an owl-o-rama. We saw great grays on the edge of dense stands of mature aspens, on clumps of willow, on fence posts, in the middle of pastures, and on the roof of an old granary. I have been a lifelong owl watcher, but I had never seen such an abundance of great grays. At one point, I stopped the car at an intersection near Opal and saw six great grays in one clearing.

Nearly three feet tall, great grays are among the largest of owls, but a good portion of their seeming bulk is composed of an underlayer of loose feathers that trap air, a good insulator. Despite their stature, they are not as heavy or powerful as great horned or snowy, owls, and they specialize on small rodents, the meadow vole being their favored prey. Great grays live in coniferous, deciduous, or mixed forests of northern North America, Europe, and Asia. Within these usually dense forests, broken treetops or the stick nests originally built by hawks or ravens provide the best places for raising young. For hunting, however, great grays either need or prefer clearings, including burned areas, ditches, and even clear-cuts. While they can be active at night, great grays hunt during the day in winter (owls abroad on a winter day are invariably hunting). Their acute hearing allows great grays to detect the movements of a mouse or vole under snow from thirty yards away. Zeroing in on a rodent, an owl drives a hole through the snow by plunge-diving from up to twenty-five feet in the air. It is able to punch its balled feet through snow crust thick enough to support an adult human. When an owl plows through their snow tunnels, the rodents are buried by miniature avalanches. The predator then reaches down with its long legs and sifts the snow by clenching its toes until it gets the rodent. In this way, great gray owls routinely capture voles two feet below the surface of the snow. After hunting, they retire to the cover of dense timber.

While great grays are technically considered migratory, their movements have neither the strict timing nor the hemisphere-spanning lengths of flights undertaken by some shorebirds and songbirds. Often, great grays will ride out the winter in the same boreal forests where they nested the previous summer. But sporadically, they set out in considerable numbers for the south. ("South" to a great gray owl means the relatively balmy realms of southern Canada and the northern United States.) Such irregular surges, or irruptions, have been recorded in Ontario, Quebec, New England, Iowa, Michigan, and Minnesota.

None of these owl bumper crops has been so well documented as those of the last two winters: the first in Manitoba, the second in Manitoba, Alberta, and Saskatchewan. In the winter of 1995-96, veteran owl banders Robert Nero and Herb Copeland managed to capture and band 115 great grays within an hour's drive of Winnipeg, Manitoba; the next year in Alberta, Cromie and Roper, using nothing more than a mouse lure and a salmon landing net, caught and banded 144. Nearly fifty of the latter were trapped within a six-week period in a ten-by-twenty-five-mile area northeast of Edmonton, a remarkable density for these large predators.

While rodent population explosions have long been known to lure predators to an area, we knew little, until recently, about what drove the owls from their breeding grounds, 500 or more miles away, in the first place. Deep or heavily crusted snow, severe cold, and slim rodent pickings had all been offered as explanations. The answer came from the work of Manitoba ornithologist Jim Duncan. He found that snow and cold did not motivate the owls, but that hunger did. Only a scarcity of rodents could force the birds off the breeding grounds. The numbers of meadow voles tend to rise and fall with little evidence of synchronized cycles, so that could explain the equally variable and unpredictable travels of great grays. Should a drop in the vole population follow a food-rich summer in which many owlets had fledged, a phalanx of hungry young birds would swell the numbers of mature owls leaving their forests to seek better hunting grounds. Add to these factors an unusual abundance of mice and voles causing the predators to concentrate locally, and you have an owl invasion.