American goldfinches were tested once to see how long they could maintain their body temperature when exposed to extreme cold. Goldfinches caught in summer maintained their body temperature for almost an hour, but birds caught in winter lasted six to eight hours. Why the difference?
Staying warm is the biggest problem birds in northern latitudes or high altitudes face in winter. Keep in mind that birds’ body temperatures vary between 100 ° and 110 ° F, averaging around 104 ° F.
Birds need to find reliable food supplies to sustain such high temperatures, but doing so in winter can be a daunting task. Many birds choose to migrate instead.
Those who stay in the north are also subject to a number of physical and physiological changes. The most obvious acclimatization 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 the outer contour feathers and the down feathers underneath are raised. In addition, birds use lint to further increase the insulation value of their plumage, which makes the plumage thicker and increases the depth of the heat-storing air layer next to the skin.
Birds as diverse as kinglets, titmice, bluebirds, capercaillie, and penguins go a step further to reduce the amount of heat they lose. They crowd together. If one of them can grip 30 percent of their body surface tightly against one or more neighbors, heat loss can be reduced by 30 percent. Cribs formed by both young and adult penguins are based on this concept.
Birds that perch in cavities, amid dense vegetation, or under snow reduce heat loss by warming the air around them, reducing the temperature gradient and subsequent heat loss between them and their immediate surroundings.
The downside of the small size
When physical methods of reducing temperature loss are not enough, birds need to increase their metabolism to produce heat. This is especially important in small birds, which have a higher surface area to volume (body mass) ratio than large birds and lose more heat through their skin.
However, generating heat means burning calories. To ensure that the seed-eaters in the north have enough fuel in winter, they store more of their digested foods as lipid triglycerides than as carbohydrates. The higher fat content causes the finches to weigh more in winter than in summer, but the high calorie content of the triglycerides is a key to survival.
Heat is generated by all muscle activity, especially flying. Birds also generate heat through rapid, out of phase muscle contractions – they tremble. It is now believed that most, and probably all, northern winter birds tremble fairly regularly.
Another heat generator is the cellular process known as non-shaky thermogenesis. In mammals, thermogenesis without tremors is known in special high-energy tissue called brown fat.
Since birds do not have brown fat, it was previously believed that they could not generate heat this way. Today, however, it is known that non-shaky thermogenesis occurs in the skeletal muscle of birds, particularly the pectoral muscle. The metabolic pathways and high oxygen supply that support the rapid muscle activity of flight also support this special heat generation process.
A northern cardinal searches for a seed in a snow-covered feeder on a cold Wisconsin February day. Photo by Matt Mendenhall
Of course, there are times when physical adjustments aren’t enough to prevent heat loss and heat generation isn’t enough to keep up. Under such circumstances, some birds go into freezing. Body temperature, heart rate, and breathing drop, and the bird becomes sluggish. The state can last for days, but in most cases freezing is simply a means of getting through a cold night. Birds usually recover by trembling the next morning and then actively feed to build the fat stores they need for another night.
From time to time chickadees will go into a slight freeze, while many hummingbirds will freeze every night. Extreme freezing occurs in goat lollipops. The common poorwill is the best example; It hibernates near hibernation for about three months.
Ducks, gulls, and other birds that spend time on ice have the added problem of losing heat through their legs and feet. Fortunately, they exit the egg with a countercurrent heat exchange system that would make an engineer proud.
Blood enters your legs at body temperature and becomes cooler as it flows down the main artery to your toes. On the way back to the body, the peripheral veins narrow and direct the blood to larger central veins near the artery. The transfer of heat from the warm arterial blood to the cool venous blood causes the venous blood to enter the body close to body temperature.
Birds that winter in northern climes or at high altitudes have to maintain high body temperatures. Physical and physiological adaptations allow them to overcome even extremely low temperatures and again demonstrate their amazing behavior.
This article was first published in the Amazing Birds column in the February 2006 issue of Birder’s World magazine.
More from Eldon Greij: How birds keep their legs and feet warm
To survive cold nights, hummingbirds cool off to record temperatures
The photo at the top of this article was a finalist in our 2019 bird portrait competition. See all finalists here.
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