Birds have developed some remarkable adaptations to accommodate cold and severe weather. Look outside on a blustery winter day and you may still see songbirds flitting around a feeder and ducks swimming in an icy creek. Nearly a million waterfowl fly to the Chesapeake region from northern breeding grounds each year. The winters here suit them just fine.
Many birds, of course, leave the Chesapeake to overwinter in warmer areas. Approximately 340 species of birds migrate to tropical regions in Mexico, Central America, South America and the Caribbean. They are known as neotropical migratory birds and include raptors, waterfowl, shorebirds, and songbirds. Each spring, these same birds travel back to the Northern Hemisphere to breed. They carry out the biannual ritual of migration using a variety of cues to help them know when and where to go. Length of day, different types of light, geographical landmarks, star patterns and the earth's magnetic field help to guide these nomadic birds.
Some birds, known as residents, remain here year-round. How do they do it? Birds, like mammals, are warm-blooded animals, meaning they maintain a constant body temperature as the temperature around them changes. To do this, they must spend their time eating 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 a primarily 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 of the obvious features that sets birds apart from other animals is the presence of feathers. Birds' bodies are covered with an outer layer of fairly stiff but flexible contour feathers and an underlayer of fluffy down feathers. The contour feathers provide protection against wind, rain and snow while 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 on their rump at base of the tail. Secreted oil is rubbed over the feathers with the beak or bill. This is known as preening. Preening conditions the feathers, creating a shield on that helps to block wind and repel water. Some birds, such as waterfowl, 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 areas of the body lose heat. The same is true for birds. But they can adjust to this in several ways. Birds will often 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 their legs, the arteries and veins in the 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. As cold blood returns to the body core, heat moves byconductance 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 reduces 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 do love their birds. A U.S. Department of Interior survey conducted in 1991 reported that 63 million Americans enjoyed backyard bird feeding and that $2.5 billion is spent annually on bird seed, feeders, baths and nesting boxes. Even if you're not an active bird watcher, when you think about the many ways that these delicate animals are able to adjust, it's hard not to be amazed.