It's barely dawn on this typical frigid January morning. I sip my coffee, trying to decide just what I'll need today to fend off the cold today: sweater and windbreaker, fleece or down jacket. I watch the cardinals and chickadees at my feeder. The frosty morning air doesn't seem to bother them at all. I decide on the down jacket.
Birds have developed remarkable adaptations to survive cold and severe weather. Look outside on a blustery winter day and you'll still see songbirds flitting at feeders, and ducks swimming in icy creeks. Nearly a million waterfowl fly to the Chesapeake region from northern breeding grounds each year. The winters here suit them just fine.
Many birds, about 340 species, leave North America to winter in the tropical regions of Mexico, Central America, South America and the Caribbean. But plenty of birds remain here year-round. How do they survive?
Birds, like mammals, are warm-blooded animals, meaning they must maintain a constant body temperature as the temperature around them changes. To do this, they spend much of their time feeding so they can generate enough heat. It's a vicious cycle though; they must eat to keep warm so they can gather more food. Birds that can switch from an insect diet to a seed diet can stay put throughout the winter. Some meat-eating birds, like hawks and owls, may also remain if enough prey is available.
One feature that sets birds apart from other animals is their feathers. Birds' bodies are covered with an outer layer of fairly stiff but flexible contour feathers and an under layer of fluffy down feathers. The contour feathers provide protection against wind, rain and snow. The down feathers act as a layer of insulation.
Tightly knit together and overlapping, feathers protect the skin and hold a layer of air over the bird's body. Because birds control the position of their feathers through muscular movements, they are able to "puff" themselves up. By adjusting their feathers, birds create and trap larger pockets of warm air near their skin, enhancing insulation.
Most birds have an oil gland located at base of their tail. Secreted oil is rubbed over the feathers with the beak or bill. This is known as preening. Preening creates a shield that helps block wind and repel water. Birds like ducks, geese and swans can survive in water that is close to freezing because the amount of oil in their feathers makes them waterproof. Waterfowl and other waterbirds also have a layer of fat that keeps them warm.
Anyone who has ever gone outside on a cold, windy day without a hat knows that uncovered body parts lose heat quickly. The same is true for birds. But they can adjust to this in several ways. Often, birds will stand on one leg, tucking the other up among their feathers. Birds are also observed with their beaks tucked under their feathers. Smaller birds often drop on the ground to cover both legs with their fluffed-up bodies.
To minimize heat loss from legs, the arteries and veins in legs of many birds lie in contact with each other and function to retain heat. Arterial blood leaves a bird's core at body temperature while venous blood in the feet is cool. Heat is conducted from the warm arteries to the cool veins. Arterial blood reaching the feet is already cool and venous blood reaching the core is already warm.
Waterfowl have fleshy feet with little blood circulation so they are less sensitive to cold. Constricting blood vessels reduce the amount of blood flow to the feet at low temperatures. Thus, the core temperature of a duck or gull standing on ice may be 104 degrees Fahrenheit but the temperature of their feet may be just above freezing.
All of this may seem of little importance as you bundle up in multiple layers to face another wintry day. But just imagine how the dull the landscape would be without these hardy souls. Winter songbirds, waterbirds and waterfowl are often the only wildlife one sees at this time of year.
And Americans love their birds. In fact, the 2006
National Survey of Fishing, Hunting, and Wildlife Associated Recreation, conducted by the U.S. Fish and Wildlife Service, found that 55.5 million people reported feeding wild birds, making it the most popular wildlife-watching activity.
As I scrape the frost off of my windshield. I'm envious of the many ways that these delicate animals are able to adjust. Then I run back inside to look for my gloves.