Alaska's Migratory Birds Weave the World Together
After Scott Weidensaul's Living on the Wind
Imagine waking up to a May sunrise and hearing a bright new song in the back yard. It takes a sharp eye to pick out the singer, a miniature bird with a voice as brilliant as its gleaming golden feathers. This turns out to be a Wilson's warbler, flitting between branches and snatching tiny insects, singing almost constantly, as if the bird is filled with all the energy of the new season.
Who could possibly imagine that this tiny creature—no longer than a child's thumb and weighing slightly more than a 25 cent coin—has just flown entirely on its own, through the vast empty skies, encountering clouds and gales and rainstorms, following the sun and stars, more than 4,000 miles from its wintering grounds in the tropical forests of Central America.
This common little backyard singer is not just a bird…it is a miracle.
Every spring, 5 billion birds wing their way inexorably northward towards breeding areas in the United States and Canada—big clattering flocks of sandhill cranes congregate at stopovers in Nebraska, California, and Wisconsin; V-shaped skeins of Canada geese pass overhead, accompanied by a chorus of nasal honks; loons and hawks, shorebirds and sparrows, thrushes and warblers…often passing invisibly in darkness and appearing in our backyards at dawn. They come in large waves and in smaller bunches, pulled by urges evolved over millions of years. Bright and beautiful migrating birds span the oceans and cross the continents, weaving our world together
Alaska is the destination for many of these travelers. In fact, about 200 species of birds travel to Alaska just for the short summer breeding season.
Some fly unimaginable distances, like bristle-thighed curlews whose migratory journey brings them north from as far away as the Marshall Islands in the South Pacific—a non-stop flight of 6,000 miles. Whimbrels fly 5,000 miles from Central America, also non-stop to Alaska. And Peregrine falcons that winter at the tip of South America journey 8,500 miles to their nesting grounds in the arctic.
Why do birds undertake these feats of endurance and navigation that challenge the human imagination?
People have been aware of the seasonal appearances and disappearances of birds, undoubtedly since the time of ancient hunters and gatherers. The earliest known depictions of migratory birds come from about 5000 years ago, when Egyptians painted images of seasonal travelers like greater white-fronted geese.
Early scientific theories explaining the autumn disappearance of birds seem simplistic today, but they were based on limited observations by place-bound people. The Greek naturalist and philosopher Aristotle thought swallows hibernated for the winter and that some birds turned into a different species of bird and then back again depending on the season. A later theory purported that some swallows hibernated in large groups under lake ice. As late as 1703, a "new" scientific theory arose which claimed that birds migrated to the moon for the winter.
Long and careful observations of bird behavior, together with recent advances in tracking capabilities, have helped to reveal the fascinating lives of migratory birds. And the more we know, the more we appreciate the truly astounding feats that happen twice each year, as birds in the billions traverse the vastness of the skyways.
We know, for example, that many birds make their epic journeys to Alaska's arctic tundra and interior boreal forests because of the unparalled conditions for breeding and raising young. Part of the attraction is a spectacular explosion of summer plant life and insects, especially mosquitos—perfect little protein packets to feed the ceaselessly hungry nestlings. Besides the abundant food, there are mild temperatures and twenty-four hour daylight, so the birds can be constantly active. Also, compared to the settled and urbanized world farther south, there are pristine environments, plenty of space to set up territories, and relatively few predators.
The growing season is short—birds have to mate, incubate their eggs, hatch their chicks, feed them as much as possible, and fatten themselves up for the coming migration back to their winter homes—and this is why the abundance of food, much of it rich in protein, is so important.
Considering their feather-thin insulation, it makes sense to assume that cold temperatures and winter gales force many birds to abandon Canada, Alaska and the northern states. But while harsh weather may trigger birds to start their fall migration, the real driving cause is disappearing food.
Loons who nest on northern lakes and eat small aquatic creatures have to leave because their food is locked under the ice for up to nine months of the year. Some loons only move as far as the coastal areas in Alaska and some go farther south to coastal Washington, Oregon, and California.
Robins may not have to move terribly far either, compared to their relatives who make marathon flights to the tropics. The robin's winter diet is mainly fruit and other plant foods, but with the approach of spring they shift to insects and especially earthworms, as they move north with the thawing ground. Robins live all year round in warmer coastal areas and throughout the southern U.S., as well as northern Mexico. Areas where soil temperatures remain above freezing, at least during the day, are especially attractive for wintering robins.
Some birds like Swainson's thrushes and yellow warblers—which sing their lovely, ethereal songs in the boreal forest each summer—travel very long distances to find suitable winter habitat. The beautiful yellow warblers winter in Central America; and many Swainson's thrushes go to the tropics of South America or even as far as Argentina.
While billions of birds and their young head out before the cold and snow and frozen waters lock up their food supply, some actually stay for the long winter. These hardy residents are specially equipped to survive some of the harshest winters on the planet, and the benefits of staying put outweigh the costs of migration…so they tough it out.
Willow Ptarmigan, for example, have feathers on their feet and legs—like a combination of mukluks and snowshoes—keeping their feet and legs warm and helping them to walk without sinking into the snow. At night or during frigid blizzards, ptarmigan dive into the snow and tunnel down to make a snow cave, where the temperature is far warmer than the sub-zero air above.
Gray jays have thick, fluffy feathers which keep them warm and make them look puffed-up compared to their southern relatives, the Stellar's jay and blue jay. All summer long, gray jays cache food in hundreds of crannies on tree trunks and among the branches, and these clever birds have an amazing ability to remember where to find each tidbit when fresh food becomes scarce during the winter.
Boreal chickadees are midget songbirds about 5 inches long and weighing only half an ounce. You might look at a chickadee and think, surely this little nothing of a bird has to go south for the winter! But the chickadee is one tough little critter, whose winter survival strategies include dense plumage, the ability to store fat quickly, a natural slowing of body processes at night, and a habit of huddling together with other chickadees inside hollow logs or tree trunks on cold nights.
Chickadees depend on stored food too, but they also eat energy-rich seeds or nuts, they find hibernating insects under bark and in crevices, and they even scavenge on dead animal carcasses. Ptarmigan are mostly plant eaters, but they get along on the meager provisions of winter—mainly twigs, seeds and buds of trees, plus whatever plant material they can scratch out of the snow.
For birds who don't migrate, the challenges of staying around during the winter are offset by the advantages: they avoid the danger and energy cost of long distance travel; they have the longest possible breeding season, and they can claim the best territories. By sticking around, they have their areas well established and families on the way before migrating birds show up in the spring.
For birds that do migrate, whether a short distance or an epic journey, every place where they spend time is critical for the survival of their species. This includes their summer breeding grounds, their wintering areas, and all the stopovers in between.
It's amazing that birds have mastered the skills needed either to survive bitter northern winters or to make vast journeys covering many thousands of miles. Whichever choice they make, their success is a true natural miracle.
Weidensaul, Scott. Living on the Wind: Across the Hemisphere with Migratory Birds.
New York: North Point Press, 1999. Print.
Alaska Department of Fish and Game
American Bird Conservancy
Audubon Water bird Conservation: Issues and Answers
H.R. 1643 Federal Bird Safe Building Act of 2011
Migratory songbirds and cats
Nature Conservancy's Migratory Bird Program
Shade Grown Coffee and Migratory Birds
The State of the Birds 2011
US Fish and Wildlife Service
USFWS Migratory Bird Program