As days get shorter and cooler in the fall, birds that stay year-round begin preparing for a long, cold winter, while others get ready to migrate. Both face challenges that seem daunting for such slight creatures. Yet birds continually amaze us with their ability to survive the harshest weather, travel incredible distances, and navigate to faraway places.
Some birds are permanent residents, living year-round in one place, while others migrate twice a year, traveling between winter homes and summer breeding ranges. Migrating is an instinctive behavior, present in some species and not in others. Whether resident or migratory, each species has behaviors and abilities suited to its way of spending the winter.
Wintering in the north, resident birds must contend with bitter cold weather, diminished food supply, and shorter days in which to forage for food. Finding shelter, wearing extra layers, and eating energy-rich food help people in cold weather, and these strategies help birds too. Many birds take shelter in dense evergreen foliage, while others sleep in hollows in tree trunks. Ruffed grouse dive or dig into snowbanks, hollowing out cozy dens in the snow. When alarmed, they burst out explosively, confusing their predators and startling many a cross-country skier! Golden-crowned kinglets, tiny birds that feed exclusively on insects even in winter, depend on companionship for getting through long, cold nights. Members of the flock call to each other as they settle in for the night, then huddle together to share warmth.
As to insulation, birds have the best kind there is. The fine, hollow barbs of a bird’s downy feathers intermesh to create tiny air spaces that trap warm air and help to conserve heat. Birds can fluff up their feathers for added insulation, and many grow extra feathers for winter; goldfinches double the number in their coats.
Small birds must eat all day long in winter to maintain their core body temperatures. Ruffed grouse fill their crops with spruce buds and then digest them during the long winter nights. Blue jays can carry five acorns at a time in their expandable throats, later burying them individually. The industrious black-capped chickadee leaves nothing to chance, hiding thousands of seeds one by one in bark crevices, clusters of pine needles, hollow fence posts – any nooks and crannies it can find. These emergency supplies are critical when the seed crop gets buried under a snowfall, or when we forget to fill our feeders. How do chickadees remember where they are? In the fall, they enlarge the memory region of their brains, adding thirty percent more cells, to help them store this information.
Swallows lined up on power lines, strings of geese honking overhead, flocks of birds lifting from the trees – these sights remind us of the many birds, two-thirds of our summer residents, that migrate in the fall, to return the following spring. Migration is the seasonal movement of birds between breeding and wintering grounds. Some birds travel only short distances, like bobwhite quail, which move from high elevation to low for the winter, and some, like red-wing blackbirds, migrate a few hundred miles. But many birds travel thousands of miles every year, between completely separate breeding and wintering ranges. Barn swallows, bobolinks, and osprey migrate to South America, wood thrushes and red-tailed hawks to Mexico and Central America, and arctic terns, champions of long-distance migration, from north to south pole and back every year. No matter how far they travel, birds return to the same locations with amazing fidelity, often occupying the same tree, nest box, or barn year after year.
Complete migrators are species in which every bird always migrates, like phoebes, orioles, and swallows. In other species, like robins, some individuals migrate and others don’t. These are called partial migrators. In some cases, only birds living at the northernmost part of their species’ range migrate. If they bypass areas occupied by the non-migrators and end up south of them, this is called leap-frog migration. Irruptive migration refers to birds like pine siskins and redpolls, which migrate south in large flocks only in years when there is low seed production in the north.
How and why migration evolved is a question still up for debate. After the glaciers receded, some tropical birds may have spread north to breed, profiting from the rich supply of insects available in northern summers. Northern species may have moved south to find food in winter. A recent study finds that the ancestors of most of our migratory birds were northern birds that spread southward. The risks of migrating are great – more than half of birds fail to return home – but the energetic benefits of ample food in winter must outweigh the costs on the whole.
For migratory birds, shorter days and cooler temperatures trigger changes in hormone levels. Birds become restless and hungry, feeding more and building up body fat as fuel for their journey. Some birds double their weight before leaving on migration, then burn all this and more on their long flights. Birds often wait for a favorable tail wind to send them on their way, and a north wind in September finds hawk-watchers waiting expectantly at favorite lookouts.
We have long known that many birds make epic migrations, but the sophisticated equipment of today – lightweight satellite transmitters, miniature geolocators – has revolutionized the field. Orioles, wood thrushes, and tiny ruby-throated hummingbirds fly nonstop for 500 miles to cross the Gulf of Mexico. The bar-tailed godwit, a wading bird that nests in Alaska, was tracked by satellite as it flew an astonishing 7,100 miles in a nine-day, non-stop flight to its wintering grounds in New Zealand, the longest single flight of any bird. Globe-trotting arctic terns amazed scientists by traveling 49,700 miles in a year, twice the distance they were thought to cover. A diminutive bird familiar to alpine hikers in the Northeast, the blackpoll warbler, makes a journey of some 2,500 miles to central South America, including eighty to ninety hours of nonstop flight over the open ocean.
Just as incredible is birds’ ability to find their way over such vast distances. In many species, parent birds head south ahead of their offspring, leaving the young to find their way by instinct. Daytime migrators like hawks and swallows use landscape features such as mountain ranges, coastlines, and rivers that run north-south. They also use the sun to orient themselves and can see polarized light on cloudy days. Rising warm air currents over mountains give hawks and eagles a lift, and they use these thermals to gain altitude, and then shoot southward to catch the next thermal. Some raptors, most notably broad-winged hawks, form kettles when they migrate. Kettles are large groups of hawks that spiral high in the sky as though stirred up with a spoon. On days with favorable winds in the fall, thousands of hawks may join kettles and fly south en masse.
Surprisingly, most songbirds migrate at night, lifting off at sunset and using the setting sun as a guide. Migrating at night has advantages, for the air is cooler and less turbulent and there is less danger from predators. Many birds rest and refuel in the daytime when they can see to find food. Flying by night, birds use the use the stars to navigate, learning the patterns on their first flights. As with sea turtles and salmon, scientists are finding evidence that birds use magnetic orientation. They can tell their position on the globe by variations in the Earth’s magnetic field, though how they do it is still uncertain. Sounds are also important, and birds that follow the coastline listen for waves on the shore. They listen to each other too, for migrating birds call as they fly. On a still night they can be heard and identified by their calls as they pass overhead. Once they are near their destination, birds recognize their homes by sight and even by smell.
Migrating birds confront many hazards. Storms can blow birds off course or cause them to lose their way. Migrators are more vulnerable to predators in unfamiliar places. Thousands of migrating birds are killed each year by crashing into lighted towers, tall buildings, and wind turbines, possibly confused and disoriented by the lights. The biggest threat to migrators is habitat destruction from pollution or development. This is especially true for birds that use only a few specific locations in their travels. Whimbrels travel 12,000 miles a year, touching down in only four places on the globe. Red knots make only one stop on their long-distance migration – in Delaware Bay, where the crabs they eat to refuel are overharvested by fishermen. These findings tell us how important it is to consider migratory connectivity, looking at all the places a bird visits during its life cycle, in order to conserve and protect our migratory birds.
We can’t help but marvel at a puffed-up chickadee on a winter morning or a hawk soaring southward on a brisk fall day. When talking about birds, those that stay and those that migrate, we quickly run out of superlatives. The ability of birds to endure bitter winters or survive arduous journeys simply leaves us in awe.
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