Staying the winter.
The Quintessential curious naturalist Henry David Thoreau paused one cold December day in 1855 to ponder in a bleak Massachusetts swamp "the incredible phenomenon of small birds in winter - that ere long, amid the cold powdery snow, as if it were a fruit of the season, will come twittering a flock of delicate crimson-tinged birds, redpolls, to sport and feed on the seeds and buds now just ripe for them on the sunny side of a wood."
A good 139 years later, on December 24, 1993, I, like Thoreau, made note in my engagement calendar of a visit by redpolls. "REDPOLLS!" I wrote, Well, perhaps not exactly like Thoreau.
But how could I hope to compete with him? Fruit of the season - what a delightful way to think of this fluffy, frosty little finch, with its strawberry breast and a red crown or poll that flashes in the sun like a police car's bubble.
Thoreau also knew that some winters the redpolls did not come, and their irregular appearances puzzled him. After he watched a flock foraging in white birches along the Concord River, picking seeds out of catkins while hanging head-first from treetop twigs, he wrote, "Common as they are now, and were winter before last, I saw none last winter."
Redpolls are no less erratic now than they were then. The birds that visited my feeder in 1993 were the first I had seen in eight years. My excitement on Christmas Eve was soon echoed on birdwatchers' telephone hotlines from Maine to South Carolina to the Midwest. "Common redpolls are everywhere, north and south," shouted New Jersey Audubon's Rare Bird Alert message. "Redpolls have invaded the Washington metropolitan area," trumpeted the Audubon Naturalist Society's recording a few days later. It seemed as if ever): redpoll in the vast boreal forest from Alaska to Labrador, where the birds usually spend the winter stoking their internal fires with birch and conifer seeds, had fled south of the Canadian border.
Scientists have a word for spectacular mass movements of birds far beyond their customary winter range: irruption. However, as Ian Newton, at England's Institute of Terrestrial Ecology, points out,"Strictly, the term 'irruption' is applicable only to the region receiving the birds, whereas 'eruption' is often applied to the region losing them."
Irruption or eruption, the phenomenon typically involves one or more of North America's "big eight" boreal seed-eaters: the common redpoll, pine siskin, purple finch, evening grosbeak, pine grosbeak, red crossbill, white-winged crossbill and red-breasted nuthatch. To that list add the black-capped chickadee, whose large flights out of the North Country are often overlooked because everybody sees chickadees every day.
Though dramatic irruptions have been the focus of decades of study, speculation and controversy among ornithologists, much still remains to be learned. "It is generally accepted that food supply is at least part of the cause of irruptions," says University of Colorado scientist Carl Bock, though, he added, there is "great disagreement," about their frequency and about whether different species irrupt in synchrony.
But the one question that most birders want answered is when the next big winter invasion of northern seed-eaters will occur. Bird irruptions, however, are as unpredictable as volcanic eruptions.
Most North American songbirds, of course, fly south for the winter. Migration is the avian solution to the problem of a disappearing food supply. Thrushes, orioles, tanagers, warblers and flycatchers wing from temperate forests to warm tropics where insects and fruit are plentiful. When the endless winter night falls on the arctic tundra, millions of American tree sparrows, snow buntings and Lapland long-spurs depart for weedy, wind-swept fields across the northern tier of the lower 48 states. Mountaintop birds like the rosy finch of the Rockies, while they do not change latitudes, do drop to lower elevations before deep snow buries the world above timberline.
Other birds are short-distance migrants that travel only a few hundred miles from their breeding places and mingle with resident southerly populations of the same species. The gang of American goldfinches at my bird feeder is a mix of local birds that never left the area and of refugees from southeastern Canada.
Unlike eruptions, migration usually follows a predictable timetable, and individual birds return to the same breeding and wintering places year after year. The northern orioles that nest in the old sugar maple by our lane arrive from Central America around May 7 every spring. A tree sparrow that I banded in our backyard one New Year's Eve was recaptured at the same bird-feeder the next winter, on New Year's Day.
There is reason, but not rhyme, to the travels of boreal seed-eaters. Contrary to popular belief, hard winters do not drive the birds south. Although the red-breasted nuthatch and various finches consume some insect life during summer months, they feed largely on conifer cones or the catkins and seeds of deciduous trees, all of which are high off the ground. Heavy snow isn't a problem - nor is glacial cold. "Little redpolls can survive at 60 degrees below zero if they have plenty of food," says biologist Craig Benkman, of New Mexico State University.
Immune to winter's worst, then, immense flocks of boreal birds wander the Canadian forest year-round, gleaning sustenance from treetop pantries - until catastrophic food shortages propel them into the eastern United States.
"The seed crops of trees vary enormously from year to year, and in some years fail completely," Newton explains. He is considered the leading authority on finch eruptions, which occur in boreal regions around the world. "The cropping depends partly on the natural rhythm of the trees themselves and partly on the weather," he says. "Most trees require more than one year to accumulate the reserves necessary to produce fruit, and they crop at longer intervals towards the north, where the growing seasons are shorter."
For good seed production, Newton stresses, trees need warm weather in the fall, when fruit buds form, and again the following spring, when the flowers set, or the crop will be delayed for another year. Moreover, the fruiting of certain species of boreal trees appears to be highly synchronized. The result: Nearly every spruce, fir, tamarack and birch tree across thousands of square miles of forest will have a bounty of seeds one fall and virtually none the following year. Yet over the next mountain range, where the climatic patterns change, the trees might be on a totally different schedule.
"In some years the productive patches are plentiful and widespread, and in other years they are few and far between," says Newton. "The birds that depend on these seeds seem to move each autumn only until they find areas rich in food, then settle there. The distance traveled varies from year to year, according to where the crops are good. Only when their food is generally scarce do they reach the furthest parts of their wintering range as an irruption. However, the food shortage that leads to a long and heavy migration is accentuated if the birds are especially numerous at the time."
Consider the white-winged crossbill, a denizen of spruce and tamarack forests from Alaska to Newfoundland. These remarkable finches use their crossed bills to wedge open cone scales so they can lift out the exposed seeds with their tongues. The birds husk each seed before they swallow the kernel, and one crossbill will eat 3,000 seeds in a day. If there is a bumper crop, the birds breed year-round - even in the dead of winten Craig Benkman found white-winged crossbills nesting in northern New Brunswick in January when the temperature was -35 degrees F.
"Crossbill populations can reach extraordinary levels in six or eight months," says Curtis Adkisson, professor of biology at Virginia Polytechnic Institute. "Juveniles breed without molting, so you can have three generations feeding on one cone crop. The outcome is inevitable: The seed supply is exhausted, the trees are exhausted, and the birds will starve if they don't leave." Crossbills, he adds, apparently foresee the end of a bonanza. "They become very restless and prepare for migration by fattening up. It's not a last-minute bailout."
Not surprisingly, vagrant finches show little fidelity to extralimital wintering places. Businessman and amateur ornithologist Robert Yunick banded 7,946 common redpolls, 3,810 pine siskins and 2,637 evening grosbeaks at Schenectady, New York, over 18 winters. Not one of those birds returned to his banding station in subsequent years. But Yunick also captured evening grosbeaks that had been banded during previous winters in Pennsylvania, Ohio, Texas, North Carolina, Virginia and Maryland.
Bird-banding records from the U.S. Fish and Wildlife Service also dramatize the transcontinental nature of boreal finch movements. Pine siskins banded in several East Coast states reappeared in British Columbia, Washington, Idaho and California. Common redpolls captured in Quebec turned up in Alaska. One amazing journey took a redpoll from Michigan's Upper Peninsula to Okhotsk in eastern Russia, a straight-line distance of 4,500 miles.
Nor are boreal seed-eaters tethered to any particular breeding places. "How far irruptive species move northward in spring also depends on how much food they meet on the way," says Newton. The common redpoll, like white-winged and red crossbills and the pine grosbeak, breeds to the edge of the tundra in North America, Europe and Asia. Over 19 summers, the density of nesting redpolls in one birch forest in Swedish Lapland ranged between 2 and 90 pairs per square kilometer. In another study, fewer than 1 percent of banded redpolls returned to their breeding area on an Alaska river delta.
The black-capped chickadee is another story. Chickadees are year-round insectivores. In summer they feast on caterpillars, in winter they scour the bark of trees for eggs and pupae. Nevertheless, half their winter diet consists of seeds of hemlocks' and other conifers, augmented by such gourmet delights as poison-ivy berries and suet from carcasses of starved deer.
"Chickadees don't put on fat reserves like the finches," says Susan Smith, professor of biology at Mount Holyoke College in Massachusetts. "The advantage is that they are more mobile in case of a predator attack." Chickadees conserve energy in severe cold by lowering body temperature at night and entering regulated hypothermia. This reduces the amount of calories, and therefore the amount of food, that they need to maintain body temperature. They also store food, which they may relocate and use as much as a month later.
Chickadee eruptions are triggered by cone-crop crashes and by habitat destruction from clearcutting of Canadian forests, Smith asserts, but close to 100 percent of the travelers are juvenile birds. "The oldest and highest ranked chickadees stay behind if there is any habitat left," she says. The young chickadees that move south in autumn, however, never make it back home. "They try to pair up and settle down where they spent the winter," she says, "but unless there has been some disaster that killed a lot of local birds, there is no room for them. They wander in a vague northeast direction well past the time when local chickadees are breeding, and most of the poor things will perish."
Are all invasions fatal events? "Mortality probably is very high," writes Carl Bock. He analyzed 10 years of data from National Audubon Society's Christmas Bird Count and found a general but not total synchrony of irruptions among the eight boreal seed-eaters as well as a biennial pattern to their southward flights. "There would be an evolutionary advantage to synchronous seed-crop sizes within and between tree species, since this could reduce population levels of seed predators, including birds," Bock suggested.
Ian Newton demurs. "It's an interesting idea, and one that is almost impossible to disprove. We really don't know if heavy mortality occurs during irruptions. It would depend on whether the birds hit areas with good or poor food supplies."
The complexity of factors leading to irruptions leaves many questions unanswered, but one thing is certain: the pleasure that birders get from unexpected northern visitors. Pine grosbeaks - plump, rosy and astonishingly tame finches - are among our rarest winter visitors, but I found them last January plucking berries from a neighbor's crabapple trees, removing the seeds like skilled surgeons and discarding the pulp. It was the coldest day of a terrible winter, and I remembered what Thoreau had written about pine grosbeaks: "There is in them a warmth akin to the warmth that melts the icicle."
The hermit of Concord knew his irruptions.
Les Line, former editor of Audubon magazine, keeps a supply of special bird seed in the hope that winter redpolls will invade his New York backyard.
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|Title Annotation:||migration of birds|
|Article Type:||Cover Story|
|Date:||Feb 1, 1995|
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