One of the marvels of avian biology is the adaptations that allow birds to live through brutally cold winter weather. To understand the ways that birds use a multi-pronged approach to accommodate the cold, we will delve into some physics, geometry and biochemistry.

Like mammals, birds maintain a constant body temperature in the face of varying environmental temperature. In the winter, birds lose heat from their surface to the cold air or water. To keep their temperature constant, their cells must produce enough heat to balance the heat that is lost to the environment.

Virtually all of a bird’s cells have organelles in their cells called mitochondria. The organelles are used for cell respiration. The reactions that occur therein also release heat. So these tiny mitochondria act as furnaces.

Imagine you are in a cabin heated with a wood stove in winter. Like a bird, the cabin is losing heat to the outside air. You feed your wood stove to balance that heat loss. As the temperature falls, you have to toss a few more logs into the wood stove to keep your cabin comfortable.

You might think a bird would do a similar process, increasing its heat production as the temperature falls. Surprisingly, birds keep their heat production constant over a broad range of temperatures with the lower limit sometimes below freezing. This range of temperature with a constant metabolic rates is called the thermoneutral zone.

How can that be? Aren’t we violating the laws of physics? Heat will be lost as the temperature falls but birds aren’t cranking up the furnace in cooler weather to compensate.

The resolution of this paradox is a bird’s plumage. Birds can raise and lower their feathers. By raising the feathers, air is trapped and forms an insulating blanket. At the higher portions of the thermoneutral zone, the feathers are lowered to decrease their insulation to prevent overheating.

Birds appear very puffy when they have their feathers fully erected. Some years ago, I was studying a population of color-banded chickadees at the north end of Flagstaff Lake. Each chickadee had two color bands on one leg.

One morning, I began the day when the temperature was 28-below F. The chickadees were so puffed up, I couldn’t see either band. The lower band was visible when the temperature got up to 15-below and both appeared once the temperature got to zero.

Geometry is cruel to small birds. The surface-to-volume ratio of a small bird is meager compared to that ratio for a large bird. The surface is where heat is lost and the volume has all the cells whose mitochondria produce heat. Chickadees and kinglets are more on the edge than eagles or owls.

One trick that small birds use, particularly when roosting at night, is to huddle. Essentially, a number of birds convert themselves to one large bird with a more favorable surface-to-volume ratio. Imagine the surprise of a Londoner who looked in a 4x5x5-inch nest box in the morning after a particularly cold night to find 61 wrens.

The choice of roosting site can provide an incremental gain in heat balance. Roosting in a conifer or in a roost cavity reduces the amount of heat lost to the wind. Roosting against a tree trunk takes advantage of the infrared radiation that the tree emits. This same radiation causes the melting of snow around the base of a tree.

These behaviors that birds use to reduce heat loss ultimately fail when the temperature gets too low. The lower end of the thermoneutral zone is called the lower critical temperature. Below this temperature, a bird has to raise its heat production,

It does so by shivering, rapidly contracting its flight muscles. Shivering is used at night since a bird cannot shiver and fly simultaneously. A chickadee may burn 40% of its body weight in a single night. Its job in the morning is to find food to replace the fat that was burned. Talk about rapid weight gain and loss.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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