Chapter 3 On Migration   Dead Creek gently meanders in an oxbow pattern through Vermont's fertile farm country before spilling out into Lake Champlain. Along it stretch miles of land planted in feed corn, forage for the dairy cows kept in the nearby barns. Hedgerows and farmhouses punctuate the pattern of rectangular fields of grain. Occasionally a herd forages in an open pasture.   Since the turn of the twentieth century, biologists have recorded the arrival of snow geese at Dead Creek in the fall. Amateur birders flock to see clamoring geese drop from the sky. Their loud whouk, whouk, whouk ricochets across the valley as family groups of geese call to one another, land, and settle into feeding.   A mix of factors draws snow geese to Dead Creek and the Lake Champlain valley. Fields of corn stubble feed them, and there is plenty of open space that allows thousands of the white-winged birds to descend together. And a wide lake provides direction as southern-flowing water leads to coastal wetlands and marshes where the geese overwinter. At Dead Creek, the birds fuel up for the second leg of their fall migration that began in the Arctic and will end in a marshy patch thousands of miles to the south.   For two weeks I have heard geese overhead, honking to each other and flying in the classic V formation. Each time I hear them I stop and search the gray sky for flapping birds. I finally line up the sounds with the dark shapes of the birds' bodies as they move swiftly through the air. I count by tens, and estimate four hundred in one flock, nine hundred in another, only fifty in a third. I try to keep track of them as long as I can until they evaporate into the distant sky. Yet even when I can't see them, I can hear the lead birds reporting back to the flock.   Hearing geese migrate is a signal. Like a deciduous forest changing colors or the way onions sweeten after the first frost, it announces that autumn has arrived. The age-old pattern of leaving an area when local resources grow scarce has begun. This place is no longer suitable; it is time to move on.   The day is clear and cloudless when I arrive at Dead Creek in mid-October. It is close to peak migration and not uncommon to see twenty thousand snow geese in a single field. A dozen cars are parked at the refuge kiosk, and I see several people with binoculars around their necks. Others have set up cameras and tripods, hoping to capture the expanse of white birds milling and pecking. I walk to the kiosk but already know from the absence of sound that there is nothing here. There are no geese, only observers looking up into the soundless sky. The birds are late and we are asking why. Perhaps warmer conditions to the north have kept them there. Or maybe agriculture has expanded in upstate New York and southern Canada and there is more corn stubble in the fields persuading them to remain longer. It could be a combination of these factors, or neither. We do not know.   Migration is a long-established strategy in human and nonhuman worlds. Songbirds migrate to overwintering grounds when food grows scarce in the north. Monarch butterflies migrate south to hang torpid from the boughs of tropical evergreens when northern milkweed leaves brown and die back. African pastoralists, nomadic Tibetans, and Minnesotan "snowbirds" migrate annually too. All are seeking better conditions: weather, food, forage, water.   Human and nonhuman migrations are linked as adaptive strategies in response to changing conditions. As Earth warms, new migration patterns and alterations in old ones are occurring. Snow geese arrive later to Vermont's cornfields, and songbirds--wrens, warblers, and thrushes--shorten their flights south. Fish and insects are migrating differently too, seeking climate spaces that allow them to mate, flower, or otherwise behave as they do now. Organisms are adapting their migrations to a northern climate that is more suitable during winter, a season that is no longer the death sentence it once was. Some, like snow geese, are staying later in their northern breeding grounds. Others, like black brant geese, are forgoing migration altogether.   Environmental conditions are not static. They vary and organisms adjust. Over the last twelve thousand years, since the advent of the interglacial period known as the Holocene--the time when agriculture arose in human societies and large Pleistocene animals like the woolly mammoth and saber-toothed tiger went extinct--countless migrations have occurred. As glaciers retreated, animals and plants ventured north to colonize newly ice-free lands. Many species expanded their ranges once the ground and waterways were let loose from the ice. Others remained in the south most of the year but ventured north to take advantage of the flush spring and bountiful summer. Thus the evolution of seasonal migrations in animals arose as a way to increase the number and fitness of a species' offspring; southern species could increase their progeny by exploiting the fresh growth and abundant prey in northern latitudes and then return to more benign conditions for the remainder of the year.   Migration in North America is often thought of as the movement of animals from the south to the north and back again. But there are also shorter migrations that happen within the North American continent itself. Carolina wrens, for instance, migrate south, but only to Maine and Vermont after spending the summer in Canadian forests, and Arctic caribou migrate above the Arctic Circle to feed on the new young growth of tundra shrubs and birth their young before they return south to winter in Canada's northern forests.   As the planet enters the Age of Warming, migratory animals are accommodating changing conditions, the very evolutionary stimulus from which their migratory behavior arose. Biologists are observing animals as they alter their historic migrations to avoid trophic mismatch, which is being in the once-correct location but at a time when expected resources have passed. Caribou, which have timed their migration to correspond with new plant growth, are finding that the earliness of spring prevents them from feeding on the most nutri tious young leaves, the value of which lactating females pass on to their young.   Similarly, the great tit, a songbird that migrates from North Africa to the United Kingdom each summer, times its migration to feed on the larvae of the winter moth. Historically, great tits hatched in synchrony with the abundance of their preferred prey. But with spring's advance, the moth, which times its emergence with young oak leaves, has also advanced and is laying its eggs earlier. Its larvae are grown and gone by the time the great tit touches down from North Africa, leaving it with an empty plate. Like the caribou's arctic shrubs, the winter moth is responding to early spring conditions and upsetting the table before the great tit has even arrived. Species involved in these types of trophic mismatches, where migration has evolved to be synchronous with resources, are the ones at greatest risk. Paradoxically, they are also the ones under the greatest selective pressure to change.   In December 2008 brown pelicans, migrating south from the Northwest Coast to Mexico's Baja Peninsula were caught in a winter storm. Hundreds of pelicans fell from the sky. Wildlife rehabilitators combed the beaches and picked up the gangly birds as they drifted in California's waters, disoriented and dying of frostbite and starvation. The pelicans, which typically migrate out of the Northwest earlier in the fall, had remained to eat the booming numbers of anchovies and sardines, salty fishes they consumed with gusto. A windfall that became their downfall.   Pelicans have been on the planet for more than thirty million years, and their impossibly graceful bodies are a visual testament to their long evolution. The pelican evolved from the archaeopteryx, a crow-sized dinosaur with a lizard's tail and joints lined with feathers that is thought to be the link between reptiles and birds. Known only from fossils (and initially only from fossilized feathers), the flying dinosaur was discovered in a German quarry in 1860 and later described by the English biologist Richard Owen. Charles Darwin, having published On the Origin of Species in 1859, used archaeopteryx to explain "transitional forms," species that existed between contemporary times and their primitive ancestors.   Seeing pelicans today, diving as they do to dip their pouch like a fishnet, gulping back tens of fishes with one elegant catch, helps us imagine a time when Earth was altogether warmer and animals, in general, were altogether bigger. When pelicans fly, their seven-foot wingspan glides but mere inches above the ocean surface, an image that evokes birds of mythical lore. Like horseshoe crabs, pelicans have evolved curiously little since they first originated. In a world where extinction characterizes life's diversity as much as adaptation, pelicans represent those left standing, the ones that have not succumbed nor become greatly differentiated in response to the vagaries of changing climates. Perhaps the Age of Warming will change all that.   ------   Baja's Pacific coastline is quieter than the rocky landscape along the Sea of Cortez on its eastern shore. Salt flats extend for kilometers when the tide is low, reflecting the near-constant shimmering sun. Shorebirds by the tens of thousands flock to these salty flats, rich in aquatic plants and marine invertebrates, to feed on eelgrass, crabs, shrimp, and mussels. They drop from the   sky, legs extending from an aerodynamic tuck near the belly. Wings flutter, ready to fold back into standing or walking pose. I look out over the mixed flock and see black-bellied plovers, curlews with strangely curved bills, and quickly moving sanderlings, petite birds that chase the retreat of waves, pecking at sand fleas and brine shrimp left behind on the wet sand.   As the shorebirds move south, lumbering among them are geese. Pacific black brant are also making their way to Mexico to feed on eelgrass, Zostera marina. Brant are dark, stout geese with black heads and matching black necks decorated with a delicate fan of white feathers. Their bellies are white with tawny brown edges, and like other geese they are gregarious and fly low in ragged flocks.   When black brant hatch in the north, biologists tag some with hard plastic bracelets on their legs and necks. Engraved with a unique set of colors, numbers, and letters, the tags can be read from a mile's distance with a powerful telescope. The information imparts where the bird was born, who its family members are, and a host of statistics specific to the individual bird gathered by researchers (and hunters) as they sight the bird throughout its range. As with Boutin's arctic squirrel, collecting long-term data on a species informs us about how animals' behavior is changing as the planet warms. For a migratory goose, biologists ask: will brant still migrate when their summering grounds remain habitable in winter? Like the pelicans, might they get caught in warm and then suddenly cold air?   David Ward, a bearded man with soft brown eyes and a slight build, is most at home in the wetlands of North America. He is a biologist with the US Geological Sur vey in Anchorage, Alaska, and has been studying Pacific black brant for decades. He has banded them in their nesting grounds in the Arctic, and then followed them south to the largest eelgrass bed in the world, the Izembek National Wildlife Refuge in Cold Bay, Alaska. Here brants gather in the tens of thousands and prepare to fly south. From Cold Bay, Ward follows them down the Pacific coast to sheltered bays along the Baja shoreline until the geese reach their southern terminus in Sinaloa on the Sea of Cortez.   Ward knows brant geese the way others know their household pets. He can distinguish between calls that signal predators and those that alert the flock to "lush eelgrass ahead." In 2009 Ward published a paper titled "Change in Abundance of Pacific Black Brant Wintering in Alaska: Evidence of a Climate Warming Effect?" in which he showed, surprisingly, that for the first time in his forty-year study of brant populations, some are no longer migrating. More and more geese were staying north, overwintering in the shallow subarctic bays where eelgrass now grows even through the harsh conditions of an Alaskan winter.   Brant geese living along the Pacific coast (there are Atlantic brant that migrate along the eastern seaboard) follow what David Ward calls "the green wave" as eelgrass beds become productive and the birds seek them out. For years the geese would breed in the high Arctic and then begin a southward migration, fueling up on eelgrass at the Izembek refuge before they took their nonstop flight across the Gulf of Alaska, a two-thousand-kilometer trip. But with the Arctic warming and the number of days that ice covers the bays falling, the eelgrass remains edible later into the year. The brant began "reconsidering" their migratory impulse. Populations made up of family groups--parent geese traveling with their juvenile offspring--began "short stopping," gambling that they could reserve their resources and withstand an Alaskan winter rather than make the energetically taxing southern trek. In essence, the geese were displaying a plastic response by greatly shortening a migration most researchers consider thousands of years old.   There are two factors that help explain why the black brant come together in the hundreds of thousands at Izembek. First, Izembek contains the largest eelgrass bed in the world, and the eelgrass there is more nutritious than in Mexico. Second, the Aleutian low, a low-pressure system that brings southern winds to the region, has historically sent brant scurrying out of the cold Arctic. There's nothing like a strong tailwind to launch a transoceanic flight! Typically, the Aleutian low is centered at 55 degrees north latitude, the same latitude as Izembek. But as the Arctic warms, the Aleutian low shifts northward into the Bering Sea. Now it lines up with brant migration only sporadically; it is no longer a dependable clue for a bird sorting through environmentally crossed messages like too many lights blinking on a switchboard.   At its least complicated, adapting to climate change means finding ways to adjust in the face of changing local conditions. Migration, and its inverse, not migrating, can be the agent of adjustment. Geese that no longer migrate are responding to easier winters. They are overwintering in regions closer to their breeding grounds rather than expending energy to fly. While not migrating carries risks--ice can freeze the bays where eelgrass grows--evolutionary theory predicts that selection will favor birds whose behaviors best fit their circumstances. If remaining in Alaska is the fittest strategy, selection will favor the ones who choose it. Alternatively, if migrating only part of the way to Baja conserves energy and hedges against variable winter conditions, we may see brant wintering in, say, Northern California. Here too natural selection will act.   Salmon migrate from the ocean to the headwaters of rivers and streams, where they breed and spawn. Freshwater eels do the reverse: they journey from inland waters to breeding grounds in the Sargasso Sea, a region in the middle of the North Atlantic Ocean. Geese, salmon, and eels are among the thousands of species whose migration patterns crisscross Earth each year, like Manhattan commuters on the maze of subways, attempting to be in the right place at the right time. Now, shifts in terrestrial, aquatic, and oceanic landscapes cannot be pegged to changes in light or length of day. The differences between the seasons are blurring, and while many species still respond to day length, others are finding that the waters are still warm, the leaves still on the trees, and their prey are still swimming. Why go? From the Hardcover edition. Excerpted from Finding Higher Ground: Adaptation in the Age of Warming by Amy Seidl All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.