From feathers to fat, birds have multiple strategies for keeping warm when the mercury dips.
BY DAISY YUHAS
Published: 12/12/2013
Each autumn as many birds begin epic journeys to warmer climates, there are always some species that stay put for the winter. These winter birds have a better chance of maintaining their territory year-round, and they avoid the hazards of migration. But in exchange they have to endure the cold.
Like us, birds are warm blooded, which means their bodies maintain a constant temperature, often around 106 degrees Fahrenheit. To make enough heat, and maintain it, they've evolved many different strategies--some similar to our own.
Sparrows, for example, seek out shelter in dense foliage or cavities to avoid the elements. They also huddle, bunching together to share warmth, and try to minimize their total surface area by tucking in their head and feet and sticking up their feathers. Cardinals, impossible to miss against the snow, and other smaller birds puff up into the shape of a little round beach ball to minimize heat loss.
"Big birds, like geese and grouse, do what we do," says physiologist David Swanson at the University of South Dakota. "They put on insulation." Their insulation often involves growing an extra set of insulating downy feathers.
Birds can also put on fat as both an insulator and energy source: More than 10 percent of winter body weight may be fat in certain species, including chickadees and finches. As a result, some birds spend the vast majority of their daylight hours seeking fatty food sources, making feeder food even more precious for surviving a frosty night.
When asked which birds are toughest winter survivors, Swanson points to little ones like chickadees. These small creatures can't put on too much bulk for aerodynamic reasons. Instead, explains Swanson, they are experts in shivering. This isn't the familiar tremble that mammals use to generate heat. Birds shiver by activating opposing muscle groups, creating muscle contractions without all of the jiggling typical when humans shiver. This form of shaking is better at retaining the bird's heat.
Another adaptation shared by many species is the ability to keep warm blood circulating near vital organs while allowing extremities to cool down. Take gulls. They can stand on ice with feet at near-freezing temperatures while keeping their body's core nice and toasty.
Keeping warm when the sun is up is one thing, but few winter challenges are more daunting than nightfall, when temperatures drop and birds must rely on every adaptation they have to survive their sleep. Some birds save energy by allowing their internal thermostat to drop. Hummingbirds are a famous example of this, undergoing torpor nightly as their body temperature drops close to outside temperatures. But torpor is not too common in winter birds, because the morning warm up would take too much extra energy. Instead, black-capped chickadees and other species undergo a more moderate version of this, reducing their body temperature as much as 22 degrees Fahrenheit from their daytime level in a process called regulated hypothermia.
One simple way to help birds when the weather outside is frightful is to hang feeders. To attract a diversity of birds, select different feeder designs and a variety of foods. A tube feeder filled with black oil sunflower or mixed seeds, for example, will attract chickadees and finches. Woodpeckers devour suet feeders. And a safflower or sunflower-filled hopper feeder entices the usual visitors plus larger birds like cardinals and red-winged blackbirds. (Check out the Audubon Guide to Winter Bird-Feeding for tips.) The birds benefit from the backyard buffet, and you'll have a front-row seat to numerous species flocking to your plants and feeders.
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Daisy Yuhas
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