Survival during frigid times
By Robert A. Hedeen, Naturalist
When the temperature was hovering around zero and snow covered the ground last winter, did you ever wonder how our little friend, the black-capped chickadee, managed to avoid freezing to death and was able to locate enough food to provide sufficient energy for survival?
During harsh times, these friendly little birds with the black, white, and gray featherswith the black incorporated into a distinctive cap and bibare frequent guests at our bird feeders. They live year-round in wooded areas from Alaska through Canada and across the northern states. Since they do not migrate south when the temperature drops below freezing, they have developed an amazing set of adaptations that enables them to effectively combat adverse conditions.
The normal body temperature of a chickadee is about 104 degrees F., so when the temperature of the environment drops below freezing, the physiological, physical, and social adaptations for survival must come into play.
The first line of defense against hypothermia is the chickadees feathers. Only birds have feathers, and they are remarkably efficient insulators of the body, trapping air between them and acting as a barrier between the birds body and the outside. The insulating quality of the feathers may be increased further by fluffing them up as the thermometer drops. About the only places on the birds body where considerable heat is lost are around the eyes and the beak.
Chickadees have no feathers on their legs and feet to prevent heat loss, but, fortunately, they are able to turn this apparent liability into an advantage by controlling the amount of warm blood that flows into the extremities. A mesh of arteries serves as a heat exchanger and effectively regulates the loss of heat to the environment
Warm, outgoing, arterial blood is brought in close contact with colder venous blood returning to the heart. This countercurrent mechanism ensures that enough warm blood will enter the legs and feet and keep them from freezing, but not so warm that there is an excessive loss in precious heat energy. In addition, the legs of the chickadee are toothpick in size, thus reducing the surface area for heat loss. Black-capped chickadees are not the only birds that use the heat exchange mechanism to keep their feet warm in winter as several other species employ the same method.
To maintain a constant body temperature, birds and mammals must take in a lot of energy-rich food, especially in cold weather. As the days grow shorter and the weather colder, chickadees and other winter resident birds must increase their food intake. Chickadees are active only during daylight hours, so in the dead of winter, they have only a short time to forage for enough food each day to supply the heat calories they will require to survive. Seeds, insect grubs and larvae, and bits of fat collected from dead animals during the fall and stashed away under the bark of trees, are reclaimed and form a major portion of the birds diet at this time. Fat is the preferred food during cold weather, as a gram of fat will yield twice as many calories when metabolized as a gram of carbohydrate or protein would.
Switching to a major diet of seeds in winter has two advantages for the chickadee. Seeds are often found in clumps, and this conserves the amount of energy required in foraging, and seeds have a high fat content.
When passing the night in freezing weather, the bird must not only have enough energy stored to provide the heat calories to survive the night, but there must be a sufficient energy reserve left by morning to enable it to begin seeking food for the coming day.
Chickadees and other birds conserve energy during the night in several ways. One method is by going into a state of torpor, or suspended animation, resulting in a much lower rate of metabolism. Torpor is an emergency procedure that appears to be employed when there is a dire shortage of food and extremely cold weather.
Frequently during an excessively cold night, chickadees will huddle together in a protected site and share the warmth of their bodies. Other small birds also employ the huddling method to prevent excessive heat loss during the night. There is a report of as many as 50 tufted titmice being found in a ball-like mass in a hollow tree on a cold winter night.
Even with all of these adaptations, life for the black-capped chickadee and others of his kin is difficult. Mortality rates of 50 percent are not unusual for populations of chickadees passing the winter in the North. However, many of these deaths are probably attributed to first-year birds that may be excluded from protected roosting sites and feeders by older adult birds.
We can certainly help alleviate some of the trials and tribulations of the black-capped chickadee and its kin by keeping our feeders stocked with seeds and suet cakes during the cold months of the year. Could we survive without help if placed in a similar situation?
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.