Avian Chilling Out

(reprinted from The Ark, December 1997)

Thanks to greeting card artists, the birds most of us associate with the holidays are the beautiful northern cardinal and the comical black-capped chickadee. That's no coincidence—while many of their feathered brethren take off for warmer climes in search of a steady food supply, cardinals and chickadees are among many species of birds that can tolerate a white Christmas.

This should surprise us—after all, in docent training and basic biology, we learned that a small body size means a relatively large surface area, which in turn means that smaller warm-blooded animals need to eat a lot to maintain their body temperatures. Cardinals and chickadees certainly qualify as small. How do they (and other snowbirds) survive the cold?

The answer is simple and amazing at the same time—through a tremendous variety of adaptations. Let's look at some of them.

Check out the chassis.We know birds are designed for flight, but many of their structural adaptations also help to conserve heat. How many times have you pointed out to your tour groups that, while fennec foxes of the desert have large ears to allow excess heat to escape, Arctic foxes' tiny ears help them to preserve their body heat? Birds have taken this one step further—no external ears from which to lose heat. Their tails consist of feathers, not flesh. Their lower legs and feet are tendinous, meaning no exposed fleshy parts. Even their bills are made of horn rather than skin. And don't forget their feathers. Not only essential for flight, feathers, when fluffed, trap an insulating layer of air (one method our chickadee friends use to stay warm on bitterly cold days).

You could have knocked me over with a feather. Many northern species have more feathers in winter than in summer. Smaller birds have more feathers per unit of body weight than their larger compadres.

Eating like a birdYou may remember that a calorie is actually a unit of heat. Because birds have a high metabolism (high rate of heat and energy production), their foods of choice tend to be rich, e.g., seeds, insects, rodents, fruit, and nectar. Their blood glucose level is about twice that of a healthy human, which induces their higher metabolism. A bird, especially in cold weather, would not do well on a low-cal, low-fat, low-sugar diet. And, of course, some birds, like penguins (some species of which survive the extreme cold of the Antarctic), have a thick, insulating layer of body fat, but this doesn't mean they're fat, per se. Think about it—have you ever seen an obese bird?

Excuse me; is this spot taken? You've no doubt seen a branchful of dark-eyed juncos, feathers fluffed, huddled tightly together, and thought, "How sweet" or "How cute." But the juncos aren't being cuddly or sociable. They're simply joining forces to combine and conserve body heat. Chickadees, woodpeckers, and nuthatches (all common winter residents in Chicagoland) sleep in holes or hollows in trees for shelter; a dozen or more bluebirds will cram into a nesting box to sleep. Evening grosbeaks, cardinals, and crossbills roost in coniferous trees, taking advantage of densely needled branches. And birds such as ptarmigans, ruffed grouse, and snow buntings will sleep under loose snow. While that may not make sense to the casual observer (after all, snow is cold), the temperature two feet under the snow may be significantly warmer than air temperature. In Alaska, if the air temperature is -50ºC, the temperature under the snow may be a relatively balmy -5ºC. That's still cold, but a bird is more likely to survive at that warmer temperature.

Nap time. Well, not exactly, but when it becomes chilly, some species, including hummingbirds, swifts, and poor-wills, will become torpid at night—conserving food energy supplies by lowering body temperature and rate of metabolism. (Mourning doves, during bitter cold or storms when they're unable to feed, don't become torpid, but may lower their body temperatures.) Torpor is not without its dangers—a torpid bird is more vulnerable to predators since it can't react quickly to danger, and coming out of torpor requires a lot of energy.

Keep your cold feet to yourself! When it's cold, birds frequently perch with one foot tucked in their belly feathers. (Not only does this keep that foot warm, but it means that much less exposed to the cold.) Ducks will sit on ice with their feet underneath them, protecting their bodies from heat losses. Many birds tuck their horny bills into their feathers during bitter cold, and have air sacs under their skin that may keep them warm with a layer of insulating air.

Brrr. You shiver when it's cold; so do birds. Most birds, including chickadees and pigeons, shiver as a short-term adaptation to cold. Shivering converts muscular energy into heat. Of course, there's a problem with that, too—the bird has to replace the lost energy at some point by eating.

Isn't that special? Many birds have special adaptations to cold. The arteries and veins of the feet in ducks and geese lie right next to each other. The outgoing arterial blood warms the incoming venous blood, so the blood returning to the bird's core is already warmed. Penguins also have this adaptation in their flippers (wings) and legs.

Some northern finches, like evening grosbeaks, can store large amounts of seeds in their well-developed crops, thus helping them to maintain their high metabolism overnight. Just before dark, hoary and common redpolls will fill a special storage pouch in their esophagus so they can digest the food (usually a particularly rich item, like birch seeds) overnight. Their high rate of energy intake means they can survive colder temperatures better than any other passerines (perching birds or songbirds).

Birds are marvelously adapted for what our climate throws at them, but not all birds can tolerate cold—at least not for very long. Between the combination of cold and lack of food, migrating birds caught in sudden cold weather may suffer a mass die-off. Stories are told of people and animal protection organizations collecting stricken European barn swallows en masse, feeding them mealworms, and shipping them by air and rail over the Alps to sunny Italy, so they could continue their journey southward on their own. That's why we aren't doing hummingbirds a favor by providing them with food past Labor Day—if they're enticed by an easy, generous food supply to stay here too long, they won't be able to escape weather that's too cold for them. Even torpor can't save them from a chilly autumn in Chicago.

Sources:

Ehrlich, Paul R.; Dobkin, David S.; and Wheye, Darryl. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. 1988.

Perrins, Dr. Christopher M. and Middleton, Dr. Alex L. A., eds. The Encyclopedia of Birds. 1985.

Terres, John K. The Audubon Society Encyclopedia of North American Birds. 1991.

5 September 2000.
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