American Goldfinches were once tested to see how long they could maintain their body temperatures when exposed to extreme cold (-70°F). Goldfinches captured in summer maintained their body temperatures for almost an hour, but winter-caught birds held up for six to eight hours. Why the difference?
Staying warm is the biggest problem that birds of northern latitudes or high elevations face during winter. Remember, birds’ body temperatures vary between 100° and 110°F and average about 104°F.
Birds have to find dependable food supplies in order to maintain such high temperatures, but doing this in winter can be a tall order. Many birds opt for migration instead.
Those that remain in the north undergo a number of physical and physiological changes to do so. The most obvious acclimatizations, or cold adaptations, have to do with plumage. Studies show that most winter birds have 35-70 percent more feather mass than summer birds. Both outer contour feathers and underlying down feathers are increased. What’s more, birds further increase the insulation value of their plumage by fluffing, which makes the plumage thicker and increases the depth of the heat-trapping layer of air next to the skin.
Birds as diverse as kinglets, titmice, bluebirds, grouse, and penguins go one step further to reduce the amount of heat they lose. They huddle together. If one of them can pack 30 percent of its body surface tightly against one or more neighbors, then heat loss can be reduced by 30 percent. Creches formed by both young and adult penguins are based on this concept.
Birds that roost in cavities, amid dense vegetation, or under snow reduce heat loss by warming the air around them, thereby reducing the temperature gradient and subsequent heat loss between them and their immediate environment.
The downside of small size
When physical methods to reduce temperature loss are not enough, birds must increase their metabolisms to generate heat. This is especially important in small birds, which have a higher ratio of surface area to volume (body mass) than large birds, and lose more heat through the skin.
But generating heat means burning calories. To make sure northern seed-eaters have sufficient fuel in winter, they store more of their digested food products as lipid triglycerides than as carbohydrates. The higher fat content causes the finches to weigh more in winter than in summer, but the triglycerides’ high caloric content is a key to survival.
Heat is generated by all muscular activity, especially flying. Birds also generate heat through rapid, out-of-phase muscular contractions — they shiver. It is now believed that most and probably all northern winter birds shiver on a fairly regular basis.
Another heat producer is the cellular process known as non-shivering thermogenesis. Well known in mammals, non-shivering thermogenesis occurs in special high-energy tissue called brown fat.
Because birds lack brown fat, it was once thought that they were unable to generate heat this way, but it is now known that non-shivering thermogenesis occurs in birds’ skeletal muscle, especially the breast muscle. The metabolic pathways and high oxygen delivery that support the rapid muscular activity of flight also support this special heat-generating process.
Of course, there are times when physical adaptations aren’t enough to prevent heat loss and heat production isn’t sufficient to keep up. Under such circumstances, some birds enter torpor. Body temperature, heart rate, and breathing all drop, and the bird becomes lethargic. The state can last for days, but in most cases, torpor is simply a means of getting through a cold night. Birds typically recover the following morning by shivering and then feed actively to build up the fat reserves they’ll need another night.
Chickadees enter a light torpor from time to time, while many hummingbirds enter torpor nightly. Extreme torpor occurs in goatsuckers. The Common Poorwill is the best example; it maintains a near-hibernation torpor for about three months.
Ducks, gulls, and other birds that spend time on ice face the additional problem of losing heat through their legs and feet. Fortunately, they emerge from the egg with a countercurrent heat-exchange system that would make an engineer proud.
Blood enters their legs at body temperature and becomes cooler as it proceeds down the main artery to the toes. On the return trip to the body, peripheral veins constrict, shunting the blood to larger central veins close to the artery. Heat transfers from the warm arterial blood to the cool venous blood, so the venous blood enters the body near body temperature.
Birds that overwinter in northern climes or high elevations must maintain high body temperatures. Physical and physiological adaptations allow them to overcome even extremely low temperatures and demonstrate again their amazing behaviors.
This article was first published in the “Amazing Birds” column in the February 2006 issue of Birder’s World magazine.
The photo at the top of this article was a finalist in our 2019 Bird Portrait Contest. See all the finalists here.