In years when winter food is scarce in the boreal forests of Canada, flocks of birds migrate south and irrupt or “burst” into local fields, forests, and feeders. The winter of 2007-2008 has already brought flocks of purple finches, pine siskins, pine grosbeaks, and evening grosbeaks to Vermont and New Hampshire. Red-breasted nuthatches are on the move, as is the predatory northern shrike.
Many birds that normally overwinter in the boreal forest—the vast woodlands dominated by northern conifers that grow south of the tundra—will migrate south when winter food is short. Every 3 to 5 years or so, birds fly hundreds or even thousands of miles in search of food for energy and warmth. Flocks of finches scatter widely as they fly, with pine siskins traveling well to the south and evening grosbeaks sometimes wandering as far as to Florida. Some predators, such as the northern shrike, often return to the same winter location. So-called superflight occurs when large numbers from many species irrupt during the same winter over a widespread geographical range.
Avian irruptions don’t foretell a hard winter, nor are they a response to extreme cold. Northern birds are hardy: redpolls can endure temperatures of –60ºF. But that’s only if they have enough food to keep their metabolisms stoked. Whenever food supplies are low, either due to a poor seed crop or an unusually successful breeding season (leading to relative food scarcity), an irruption is likely to take place.
Because each species prefers specific foods, irruptions depend on which northern foods are in short supply. Among the different finches, crossbills eat the seeds of conifers while redpolls prefer birch and alder catkins. So finches won’t all irrupt during the same winter, and their movements often don’t overlap with those of black-capped chickadees, boreal chickadees, red-breasted nuthatches, or Bohemian waxwings.
Many birds mitigate the dangers of food scarcity by stashing food for later use. Chickadees store quantities of seeds, insects, spiders, and other foods by jamming them into bark crevices and other nooks. Gray jays or “whiskey Jacks” use saliva to glue their food to tree trunks and branches above the normal snowline.
In Life in the Cold, author Peter Marchand says that most winter birds possess enough fat reserves to survive one night of extreme cold. Birds shiver constantly to generate body heat, and chickadees lower their nocturnal body temperature by 10-12º F to save energy. Redpolls and goldfinches grow a winter coat that weighs 1 1/2 times their summer plumage.
One December day in 1981, I encountered a flock of pine grosbeaks feeding on dried crabapples. Like many boreal birds, pine grosbeaks are inordinately tame: I approached within a few feet of the sleek, rose-feathered males and elegant, gray, yellow-rumped females. Pine grosbeaks don’t travel very far south, often settling in northern states where there are enough dried berries, ash buds, and seeds. These docile birds rarely alight at the feeder, in contrast to the brightly-colored, raucous evening grosbeaks that invade en masse.
Some irrupting birds can’t be distinguished from year-round residents because they look alike. Winter flocks of black-capped chickadees, blue jays, and American goldfinches may be a melange of local birds mingled with those that migrated here from farther north.
The unmistakable snowy owl, however, only shows up as a periodic winter visitor when the northern population of lemmings plummets. For the same reason, rough-legged hawks will move farther south in greater numbers than usual; likewise northern shrikes when mouse numbers decline. The periodic paucity of red-backed voles every 3-5 years can cause irruptions of great gray owls, boreal owls, and northern hawk owls.
In early December of 1995 while taking a walk, I discovered a deer mouse impaled on the lance-like spike of a hawthorn. After catching their prey, shrikes sever the spinal column with a fatal bite and store the carcass on a thorn, in the crotch of a tree, or on barbed wire. I noticed that a new prey replaced the old one every few days. The shrike often perched nearby but refused to eat while I was watching.
Observing winter birds and their behavior is more than an interesting pastime. Citizen reports of bird sightings enable ornithologists to keep track of avian wanderings over vast regions. Many records of winter birds come from participants in the Cornell Laboratory of Ornithology’s Project FeederWatch and the National Audubon Society’s Christmas Bird Count.
Tracking local irruptions is a wonderful way to enjoy winter birds. On December 11, 1855, Henry David Thoreau wrote: “When some rare northern bird like the pine grosbeak is seen thus far south in the winter, he does not suggest poverty, but dazzles us with his beauty… these rich but delicately tinted and hardy northern immigrants of the air.”