In his book Winter World, scientist and naturalist Bernd Heinrich said something that puzzled me. He was describing the effect of the cold on small birds and wrote: “The physics of heating and cooling dictate that small objects cool quickly, since every point within them is close to the surface where heat is lost. The smaller the animal, the proportionally larger is its surface area, which is the drain whereby it loses heat.”
I had trouble visualizing his statement that the smaller the body of the creature, the higher the ratio of skin surface to body mass. I just thought a bigger body mass would also lead to larger skin surface. The body has to enclose all that mass, doesn’t it?
Now, years after reading Heinrich’s book, I’ve been thinking about the perky little chickadees we have at our feeders and wondering how they can survive our bitter winter nights. Do their little bodies lose relatively more heat than the larger blue jays and mourning doves? I still couldn’t visualize how skin surface area changed with an increase in body mass.
So, I decided to take a mathematical approach to analyzing the thesis. I thought I could approximate a bird body as a sphere and look up the formulae for the surface area and the volume of a sphere. The surface area of a sphere = 4πr², where r is the radius of the sphere. The volume of a sphere = (4/3)πr3. To get the ratio of skin surface to body mass, divide the first formula by the second one. The result is simply 3/r. Isn’t that neat?
Since the radius is in the denominator, a smaller radius gives a larger ratio of skin area to body mass. Therefore, we can estimate that a smaller critter loses relatively more body heat than a larger one.
It’s true that a larger bird has more skin area than a smaller one, but that ratio of area to volume is smaller for a larger animal.
All of this fancy math just points out the problem: If you’re a little bird, you’re going to need some strategies and techniques to make it through a Minnesota winter night.
One thing that chickadees do is add additional feathers to their bodies as winter approaches, especially the little downy feathers near their skin. It’s the equivalent of putting on their long-handled drawers as the days and nights get colder.
Like other birds, chickadees also fluff out their feathers to retain heat. Fluffing creates air pockets that insulate the body from the exterior chill.
Norwegian scientist Jon Steen studied titmice and five species of common finches one winter outside his lab near Oslo. He found that the birds had a couple of techniques for reducing heat loss. The birds would “ball up,” tucking their heads into their back feathers, thereby reducing heat loss, especially from the areas around their eyes and beaks.
But, more important, he discovered that the birds were able to lower their body temperature at night, going into a torpor-like state known as hypothermia. When plenty of food was available, they maintained normal body temperature but shivered the whole time, often while sound asleep.
Here in North America, the black-capped chickadee has been studied extensively for its rate of heat loss in winter. Susan Chaplin at Cornell University in upstate New York found that chickadees were able to lower their body temperature from their normal daytime 107˚F to the range of 86˚F to 90˚F. This helps to slow the use of their fat reserves during the night, in spite of vigorous shivering, even while they’re sound asleep.
For me, shivering is usually the prelude to teeth chattering and loss of muscle control. Once I start shivering, I can’t think about anything else but getting warm. Birds, however, use shivering as a way to generate heat. And they can do it while they sleep.
The shivering does use up their fat reserves, however. Chickadees usually start each winter day with little fat reserve and have to find sources of fat to lay on enough fat (more than 10 percent of their body weight) to get them through the coming day and especially the night ahead. We can help by providing good sources of fat at our feeders, like suet and black oiler sunflower seeds.
Chickadees often spend the night in a tight-fitting roost hole where the surrounding wood gives them some insulation from the nighttime temperatures. And they are better off if they find a hole that faces away from the night wind. Heinrich said you can tell how snug their night accommodations were by noticing that some chickadees have bent tail feathers in the morning.
They might be described as having a bad tail day, but they’ve survived another Minnesota winter night.
Clay Christensen lives and writes in Lauderdale. His book, The Birdman of Lauderdale, is available at local book and bird stores and online at BirdmanBook.com.