That's what scientists call the periodic movement of millions of finches from the North during the cold months
On a sunny April day in Upstate New York, a TV news van whipped into a shopping center parking lot. At the tables of a sidewalk cafe, people elbowed each other and pointed. Some celebrities were paying an unexpected visit.
Were they foreign heads of state? Nobel prize-winning physicists? Rock stars? Reasonable guesses in Ithaca, New York, the home of Cornell University--but wrong. No, the visitors were birds--small, reddish, sparrowlike birds with odd bills that crisscrossed at the tip.
The little flock of red crossbills and white-winged crossbills had alighted in a tall Norway spruce at the edge of the parking lot. A rare sight in New York, they were the retreating wave of a spectacular winter-long and continentwide invasion--a so-called Superflight.
Crossbills are members of a bird group that scientists call "winter finches" because they sometimes show up in unusual places in the winter. The winter finch roster also includes the pine siskin, purple finch, evening grosbeak, pine grosbeak, common redpoll and hoary redpoll--all members of the finch family. (Only three species of North American finches are not winter wanderers: the lesser goldfinch, Lawrence's goldfinch and house finch.) Red-breasted nuthatches, which are more closely related to chickadees, might also be considered honorary "winter finches" because of their similar feeding--and wandering--habits.
Winter finches normally live year-round in the boreal forests of Canada and Alaska, or high in the Sierra Nevada, Cascades and Rocky Mountains. These are birds whose lives are inextricably bound with vast dark forests of evergreen trees. They feed on tree seeds: Crossbills eat the brown seeds of conifer cones; redpolls eat the small seeds within the caterpillarlike catkins of birch and alder trees; pine grosbeaks eat buds and the seeds of ashes, along with berries from many shrubs. Winter finches are hardy birds, adapted to extreme cold and deep snow.
But some winters the finches, like rock stars, go on tour. It's not that ice and snow are a problem, it's that birds, like balladeers, have to eat. Last winter they hit the road by the millions (scientists don't know exactly how many), making headlines across the country. White-winged crossbills were spotted in Tennessee, common redpolls visited Virginia, evening grosbeaks paid a call in South Carolina and red-breasted nuthatches dropped in to Texas.
When a single species wanders out of its usual home range, the phenomenon is called an "irruption." When many kinds of winter finches irrupt simultaneously, that is a "Superflight," a term coined by ornithologist Carl Bock of the University of Colorado at Boulder. Bock had noticed general patterns to irruptions: Redpolls seemed to invade every other year, for example, and evening grosbeaks every three or four years. Beginning in the late 1960s, Bock and his students examined data from the National Audubon Society's Christmas Bird Count (a yearly volunteer-based bird survey) to see if irruptions of different species were synchronized. Did winter flights of crossbills and redpolls happen at the same time? Did the birds invade the eastern and western United States simultaneously?
"The Christmas count data supported these ideas," says Bock. "Irruptions did seem to be widely synchronized. What was especially striking was that there seemed to be certain years--kind of unpredictably--where the synchrony was stronger, where the birds came in bigger numbers and went farther south, and everyone was seeing them everywhere." During these Superflight years--1968-69, 1972-73 and 1982-83--anything was possible. Bock remembers the year red-breasted nuthatches were spotted perching on fence posts in a cemetery on the plains of eastern Colorado--hundreds of miles from the nearest tree.
The 1997-98 Superflight was documented with the help of the Internet. Steve Kelling, an avid bird-watcher and employee at the Cornell Laboratory of Ornithology, logged onto a local birder's listserv one October day and saw that several different kinds of winter finches had been sighted around Ithaca. That prompted Kelling to put in a query to a national listserv: "Anyone else seeing winter finches?"
"I got a tremendous response," he says. "Finches were everywhere--Maine, Texas, Oregon, Washington. I collected the information, put dots on a map, and realized it was going to be a Superflight."
Bird-watchers were asked to report their winter finch sightings online, using a special winter finch survey form on BirdSource, a new web site developed by Cornell and the National Audubon Society. By May, more than 8,000 online finch surveys had been filled out and submitted electronically. "Nobody really knew the dynamics of a winter finch irruption: how quickly do they travel, do they go to just one area or many places, do they stop when they find food, or do they keep wandering all winter?" says Kelling, the BirdSource project leader. Researchers are still wading through the data to find answers to most of these questions. But the weekly reports allowed Kelling to produce animated maps, viewable on the BirdSource web site, that show just how the birds moved about.
Bird-watchers also gathered information on the birds' feeding habits. More than 90 percent of all redpolls and evening grosbeaks were spotted at bird feeders. This suggests that the booming pastime of backyard bird feeding may be influencing these birds' wanderings. Fewer than 5 percent of all crossbills, on the other hand, were feeder visitors. Instead, they showed up in places where birders reported that conifers were loaded with seed-filled cones. "Crossbills seem to find the areas with the biggest seed crops and just stay there all winter," says John Fitzpatrick, director of the Cornell Laboratory of Ornithology. "This fact has been known for some time, but this is the first time anybody's been able to document the process week-by-week as the Superflight unfolded."
In Ithaca this past winter, the crossbills in the parking-lot spruce seemed quite at home, contentedly feeding among the thick clusters of brown cones. With their stout bills and thick legs, these birds look a bit like miniature parrots--a resemblance that is reinforced by the way they use their beaks like an extra leg, to cling to branch tips. They may even hang upside down while feeding, poking their crisscrossed beaks into the crevices of dry brown cones.
Ornithologist Craig Benkman of New Mexico State University has studied the workings of those scissor-tipped bills. He says a crossbill bites between the scales on a cone, then "chews" sideways, moving the lower jaw to the left or the right (depending on the direction in which the tips cross). With this motion, the bill tips pry the scales apart. That exposes the hidden seeds, which the bird laps up with a spoon-tipped tongue. The tongue holds the seeds in a groove in the upper palate while the lower bill cracks the seed coat. The tongue then strips away this clinging husk.
The design of a crossbill's bill is the key to another recent discovery. What scientists have regarded as a single species, the red crossbill, may in fact be eight separate species--or even more, according to Benkman and Jeff Groth of the American Museum of Natural History in New York City, both of whom have investigated crossbill taxonomy. Each type of bird has a bill that is subtly modified--just the right size and shape to open a certain kind of cone. Or, the palate groove that holds a seed while it's husked is shaped to fit a certain kind of seed. There's a crossbill that specializes on ponderosa pine seeds, one that's equipped to eat lodgepole pine seeds and one focusing on western hemlock. Though they look alike to the casual human observer, the birds recognize their fellows--and appear to refrain from interbreeding--because each type has a distinctly different call.
Seeds--or rather, seed shortages--are what spark a Superflight. Living where the temperature can hit minus 60 degrees Fahrenheit, winter finches need plenty of fuel. Craig Benkman has calculated that a white-winged crossbill foraging in spruce and hemlock trees on a typical winter day in southern Canada has to eat a seed every seven seconds throughout the eight hours of daylight to meet its caloric requirements. "That doesn't leave lot of spare time for flying from tree to tree, preening or socializing," he says.
When seeds are plentiful, it is possible to find one every seven seconds--but seeds are not always plentiful. "All of these cone-bearing trees are unreliable," says Curtis Adkisson, a professor at Virginia Polytechnic Institute who studies winter finches. "They put out a big cone crop one year, and the next year nothing at all. It may be two or three more years before they have another really big crop."
Biologists don't know why tree seed production is synchronized over large areas. "Trees can't talk to each other, as far as we know," says Bock. "So how do they decide this is the year they're going to make seeds, or not make seeds?" Weather or other environmental factors probably influence how much seed is set. "But it's still an unsolved mystery," he adds.
Some scientists speculate that the trees and the birds are engaged in a kind of arms race. In years when conifers have a big seed crop, the birds reproduce like crazy. (Young crossbills mature quickly and breed when they're just a few months old.) Bird populations grow quickly, and the big flocks gobble up the seeds. A low seed year may be an evolutionary adaptation that sends "seed predators" packing, allowing the trees to recover.
The unpredictability of irruptions is what sets them apart from the more familiar pattern of bird movement called migration. Migration is tied to regular, seasonal changes in day length and the accompanying, predictable changes in food availability. You can mark on your calendar when migratory birds will show up on their summer breeding grounds or wintering territories. Year after year, they follow the same routes. "You expect a migratory bird to go back to its home range to breed," says Adkisson. "With birds like red crossbills, if you have that expectation, you're going to be fooled and fooled and fooled again."
Winter finches breed wherever the seeds are. Not far from Ithaca this past April, a female crossbill was spotted in the town of Dryden, gathering nesting material in a tree on--where else?--Evergreen Street. At a time when migratory songbirds were winging to their traditional nesting grounds, this winter wanderer had, by chance, found a good spot and seemed to be settling in to raise a brood.
Crossbills don't just breed wherever seeds are plentiful. They will breed whenever seeds are plentiful--"even in the winter, in the ice and snow, when days are short," says Adkisson. "That's very unusual among songbirds." Thomas Hahn of Princeton University has studied the factors that control the red crossbill's yearly breeding cycle. Most birds respond to light and breed only when the days are a certain length--usually 15 hours of daylight or more. But as long as the cone crop is good, crossbills don't care what the weather is doing. They can raise a family in June or January--or both.
"It's a fantastic story," says Adkisson, "but it all makes sense. Crossbills are the ultimate opportunists, adapted to breed under short day length, when the food is there." These nomadic birds are champions of adaptability, ready to take advantage of a good situation wherever they find it.
Although none of the winter finch species are considered endangered or even threatened, their erratic comings and goings tend to spark public concern about declining bird numbers. Erica Dunn, a biologist with the Canadian Wildlife Service, documented plenty of winter finch irruptions as the director of the Ontario Winter Feeder Bird Survey and the Cornell Laboratory of Ornithology's Project FeederWatch, two volunteer-based bird-monitoring efforts.
"People notice when the finches are here," she says. "There's nothing like a crowd of evening grosbeaks at your feeders. They're just so big and conspicuous. But then, in years when winter finches don't irrupt, everyone worries. People say, 'Oh! This is a terrible winter for birds.' People may not realize that only certain birds are 'missing.' The year-round winter residents are still there in the same numbers."
Will those wandering finches be back this winter? Bock and other scientists say probably not--but it's hard to know for sure. If you don't see much action at your backyard feeders this winter, don't worry. Those superstars of spontaneity, the winter finches, are probably hanging in their native haunts this winter. But like that enduring rock group whose name evokes the birds' lifestyle, the winter finches are sure to make a comeback. The Rolling Stones of the bird world may gather no moss--but they're always gathering seeds.
Cynthia Berger produces radio programs about the natural world for Finger Lakes Productions in Ithaca, New York. She wrote about whale songs in the June/July 1998 issue.