Cold (14 page)

I leave campus at seven thirty in the morning. It is thirty-six degrees. Ten minutes later, I am in the Barrens. A possum
rests peacefully in the road, its gray fur dusted with frost. I open my windows, enjoying the cold. At a sign that says grouse
hunting area, it is thirty-four degrees. At a shooting range a few minutes down the road, it is thirty degrees. The forest
is still recovering from the work of woodcutters and charcoal production. The trees are uniformly young and scattered. My
left ear, exposed to the full blast of wind coming through the window, is now comfortingly numb. In the heart of the valley,
still fewer than ten miles from campus, it is twenty-six degrees, a full ten-degree drop from campus. My hands are stiff on
the steering wheel. My left ear is beginning to hurt. I turn the car around, my curiosity satisfied, ready to migrate back
to the warmth of campus.

The poorwill — to the Hopi people, “the sleeping one” — hibernates through the winter, but it is an exception among North
American birds. Another hundred or so of the continent’s birds tough it out through the winter, feeding frantically to supply
the calories needed to stay warm. The other 550 species that breed north of Mexico migrate.

A migrating bird might fly and fly and fly some more, past cities, above browning cornfields and forests, over vacant Gulf
of Mexico waters, a night and a day and a night passing in the air, a few ounces of feathers and flesh and aerodynamics, finally
landing, wasted, almost nothing left. Or if it is, say, a Clark’s nutcracker — a gray jay with a white face — it might go
no more than a few miles, like a moose moving from a mountainside to a neighboring valley. If it is a mallard, it might hop
south, moving from pond to pond one step ahead of the freeze line. Many birds stop in the southern United States. More than
two hundred species go on to the beaches and coastal forests of Mexico. Others head for the Caribbean. A few dozen make it
as far as the Amazon, overwintering with parrot friends. A few — the barn swallow, the upland sandpiper of the American grasslands,
the Swainson’s hawk — make it to the Pampas of Argentina. Some change their migratory habits from year to year, wintering
wherever they find the right balance between availability of food and the lessening of calorie-sapping cold. Great gray owls,
very much northern birds, head south only if they get hungry enough. They showed up south of their normal range in the winter
of 1978 and again in 1983. One made it as far south as Long Island. There, well outside their normal range, they were sometimes
seen feeding in broad daylight, not behaving as good owls do. They were hungry.

Aristotle believed that swallows hid underwater during the winter. He also believed that worms came from horsehair, and he
thought that the European redstart transmogrified into the old world robin. For centuries, others saw the world through glasses
tinted by Aristotle’s errors. As late as 1555, Olaus Magnus, archbishop of Uppsala, wrote of swallows, “They cling beak to
beak, wing to wing, foot to foot, having bound themselves together in the first days of autumn in order to hide among the
canes and reeds.” But Aristotle knew more than his widely publicized mistakes might suggest. “Others migrate,” he wrote, “as
in the case of the crane; for these birds migrate from the steppes of Scythia to the marshlands south of Egypt where the Nile
has its source…. Pelicans also migrate, and fly from the Strymon to the Ister, and breed on the banks of this river.” Other
ancients, too, knew of migration. Homer said that cranes “flee from the coming of winter and sudden rain and fly with clamor
toward the streams of the ocean.” The Old Testament reports “the stork in the heaven knoweth her appointed times; and the
turtle [dove] and the crane and the swallow observe the time of their coming.”

We continue to learn about migration. For example, each year the spectacled eider, one of the nation’s most beautiful ducks,
disappears from the summering grounds of the Arctic and reappears each spring. Where does it go in the winter? Prior to 1994,
no one was sure. For all anyone knew, it could have overwintered underwater, bill to bill and wing to wing. In 1994, biologists
followed the signal of a transmitter implanted beneath the skin of a spectacled eider. They found the eider, and more than
a hundred thousand of its cousins, secure in the pack ice, jammed together in a pond of open water surrounded by thick ice
and bathed in Arctic winter darkness, with air temperatures of thirty below. The eiders’ collective motion and body temperature,
it seemed, contributed to the maintenance of the open hole that was their home. They overwintered by feeding and paddling
about in that.

There is more to be learned. There are, for example, physiological adaptations. Not unexpectedly, birds put on fat, but in
some cases nonessential organs shrink. Just before migration, the bar-tailed godwit becomes fifty-five percent fat, but its
kidneys, liver, and intestines shrink. Then it flies nonstop at something like 45 miles per hour for days on end. The speed
and exact route of many birds are not known. Migrating sea ducks tracked by radar in the Arctic fly at more than 50 miles
per hour. A dunlin — a long-beaked shorebird — was once clocked at 110 miles per hour, passing a small plane. Other unknowns:
How do they cope with man-made obstructions? How do they respond to the lights and noises of cities and ships and smokestacks?
Do flashing lights warn them away or just confuse them? In 1998, migrating Lapland longspurs came upon an antenna tower in
the Kansas fog. Apparently confused by the tower’s blinking lights, they circled it, again and again and again. They ran into
guy wires, into the tower itself, into one another. Before it was over, ten thousand were dead on the ground.

Bird banding has been to avian biologists what the telescope has been to astronomers. A band — a metal or plastic tag — is
clipped to a bird’s leg, pinned to a wing, or placed as a collar around a neck. Each band has a number and an address. The
bird might be found dead, or shot, or captured by other banders, giving up information on its movements. Henry IV of England
banded falcons at the beginning of the Little Ice Age. Duke Ferdinand banded a heron in 1669. John James Audubon is said to
have been the first bander in the United States. Today a single banding station, staffed for the most part by volunteers,
might band a thousand birds in a week. The U.S. government issues one and a half million bands each year. Something like sixty-three
million birds have been banded, and just under four million bands have been recovered. A letter from China accompanying the
return of a pintail duck band reads, “I feel very glad to wrote you. I did not know you and you did not know me. Who introduced
I to you? It is your pigeon. She Flew To China. What a far way she flew! It is marvelous.” Less helpfully, other bands have
been returned with notes asking for recipes.

A returned band will say something about where a bird went but not how it got there. Bird navigation is no simple matter.
Navigation skills are both learned and instinctive. Birds follow rivers and shorelines. They use the sun and the stars. They
hear breaking surf. They may detect differences in air pressure. Some have magnetite in their nasal cavities — built-in magnetic
compasses. Experimenters have done odd things. One looked at bird behavior in a planetarium, confusing his birds by turning
off stars and star groups and entire portions of the sky. Another put electromagnets in birdcages. After all of this, nothing
is completely clear. The truth behind migratory navigation defies generalizations.

A flock of migrating birds is more than the sum of the individual birds, and less. Free will seems lost. Some combination
of instinct and memory and groupthink drives the beat of wings away from the cold and toward reliable food. Birds flew some
of these routes before men hunted with stones. Many knew the sting of the nineteenth-century market hunters, feeling the flak
sent up from shotguns mounted on the bows of punts like batteries of antiaircraft guns strung along the nation’s flyways.
Birds have seen the lights of cities flicker on. They have run into buildings that grew out of prairies. Resting in trees
or on the ground, birds have learned of the dangers of house cats: one hundred million birds feed themselves to tabbies every
year.

Most recently, birds have discovered microwave communication towers, running into them at full migratory speed, no doubt thinking,
This wasn’t here last year,
just before abruptly stopping beak-first, skull-second, bones shattered, then falling to the ground, migration finished,
game over, lights out.

It is October twentieth and fifty-four degrees in Anchorage. In my absence, the snow has crept farther down the mountains,
but for now it is still above the streets and houses and office buildings of the city. I am disappointed. It has been too
long since I last felt snow under my feet. I drive to Flattop, a little mountain just outside town with what is reputedly
the most hiked trail in all of Alaska. I am here to walk in snow, but also to look for ground squirrels, thinking that this
might be my last chance to see them before they hibernate.

Within ten minutes of the trailhead, at something like twenty-five hundred feet above sea level, I see hoarfrost, water vapor
frozen into intertwined white crystals on the surface of the ground. The willows along the trail are nearly leafless, and
the crowberries hang from stems that have turned from rich green to dry reddish brown. A bull moose forages on the slope beneath
the trail. Soon the moose will find its way into the valleys, where it will stand in the winter cold, gnawing bark from shrubby
trees. It will lose its antlers. When the leaves are gone and there is nothing to eat but bark, moose might spend six hours
feeding and twelve hours ruminating. Eating more to stay warm is pointless, because they cannot digest their food as quickly
as they can feed. They lose weight. In a bad winter, many will starve. But it must be a very bad winter. Their metabolic rate
does not increase until temperatures drop below minus twenty. As often as not, their problem is staying cool. Winter-acclimatized
moose are known to start panting as temperatures rise toward the freezing point.

Within twenty minutes of walking, there is snow on the trail — white dust scattered on the surface of dark mud and caught
in the leaves of crowberries. Here, at twenty-six hundred feet above sea level, it is ten degrees colder than at the trailhead.
The clouds open for a moment, and the sun bathes Anchorage and Cook Inlet. I pull on a hat. A few minutes later, the clouds
close up again, and big slow-moving snowflakes drift down, sticking to my shirt. At around twenty-eight hundred feet, I scan
a slope of splintered bedrock and ice-shattered boulders, looking for ground squirrels. They are common here, among the rocks.
In summer, they stand on their hind legs, imitating prairie dogs, watching tourists panting and struggling up the mountain.
The ground squirrels will overwinter in burrows hidden beneath the rocks. It is in these burrows that they fall deep into
hibernation. It is here, during winter, that their body temperature drops to just below the freezing point of water, close
to thirty degrees. The ground squirrel’s blood is more than ten degrees colder than that of a hibernating chipmunk. In the
laboratory, a ground squirrel blood sample would freeze at thirty-one degrees, but within their frigid sleeping bodies, ice
does not form. Their blood is super-cooled — it is below the freezing point, yet not frozen. Supercooling occurs when there
are no nucleation sites, no place for nascent ice crystals to get a toehold. Formation of a single ice crystal in the supercooled
blood of a hibernating ground squirrel could flash freeze the little beast. The ice crystals would rip through cell membranes.
The squirrel would die.

Here, too, in the burrows beneath these rocks, as often as every two weeks through the long cold of winter, the squirrels
warm up. They are no different from other hibernators in that they need their sleep. They burn away fat reserves to warm up
enough to sleep. An electroencephalogram of a cold squirrel shows a quiet brain, but when they warm up enough to sleep, the
same electroencephalogram will show the patterns of dreaming. Dreaming of what? Of spring? Of succulent shoots and fresh flower
buds? Of mating? For about a day, they warm up, and then they cool off again, quickly dropping to near freezing, and then
somehow maintaining their body temperature at just above freezing. How do they do it? How does a nearly frozen brain, its
neurons less nimble than cold molasses, tell the little rodent that it is time to wake up? No one knows.