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As the northern hemisphere dips into its winter angle, we wave many of our small songbird neighbors bon voyage on their migratory journeys. But there is one tiny passerine that stays here waving with us, and chooses not to migrate away from our frigid temperatures: the black-capped chickadee. Chickadees, in fact, hardly shrink from view as the winter encroaches; instead, they are bold, brazen, and positively belligerent. Bernd Heinrich writes in Winter World, “…no walk in the snow-filled winter woods is complete without at least one run-in with a family flock of these small, tame, and inquisitive birds as they search for food.”
So how do they do it? How does a bird that weighs no more than a dozen paper clips not only survive our sub-zero winter nights, but thrive?
To understand, we must first look at the physiology that keeps them on this knife-edge of survival. Like us, birds are “warm-blooded” or, more accurately, endothermic and homeothermic. As endotherms, we generate our body heat internally using the energy from our food, and as homeotherms, we keep our body temperature relatively stable across time. For humans, that temperature is anywhere between 97°F and 99°F, but for the black-capped chickadee, it is about 107°F.
Keeping the body at this high temperature when the ambient air is 0°F or colder requires an extreme amount of energy–much more energy than a chickadee’s diet alone can provide, though it can provide quite a bit. Chickadees are insectivores, and in the summer around 90% of their diet consists of insects that they glean from twigs and foliage. In the winter, their diet switches to only 50% animal protein, with the other 50% provided by nuts, seeds, and suet. They especially prefer black oil sunflower seeds, peanuts, and suet, because those items sport the highest fat content, and fat is much denser in calories than carbohydrates. Chickadees require so much fat in winter to power their bodies that they have even been observed munching on the fat of frozen animal carcasses.
Anyone who feeds the chickadees in their yard also knows that these tiny birds are rarely content to sit and stuff their faces at the feeder, but instead grab one seed, fly off with it, and come back in a few moments to snag another. Chickadees cache some of their finds for later, and are able to remember thousands of hiding places and return to them when food is scarce. Still, chickadees have been measured to lose around 57% of their body fat content on cold winter nights, a number which, while impressive, would be much higher were it not for the ultimate secret to chickadee survival: feathers.
Bill Chaisson of the Eagle Times writes,
“Feathers operate on principle similar to that of storm windows; a layer of air between the outside and the inside keeps the inside warm.”
Black-capped chickadees have much denser feathers than other birds of similar size, increasing the number of these possible pockets of warm air trapped against their skin, especially when they puff up those feathers like a golf ball. The reduced surface-area-to-volume ratio of being basically a sphere also minimizes the heat lost to the environment, except in certain areas like around the eyes and beak. That is why you often see sleeping birds with their faces tucked in the feathers of their back, between the shoulder blades–burying those areas of exposed skin in warm feathers.
The one area of exposed skin left is the chickadee’s feet, which like many other birds has its own system for maintaining core body temperature. Countercurrent heat exchange occurs in the blood vessels in the bird’s legs, and allows warm blood coming down from the body to share its warmth with cooler blood coming back up to the body from the toes. The toes themselves have so little muscle and tissue in them that they don’t require as much heat as our own toes or fingers do to continue to function.
A final strategy chickadees employ is to find a place with a higher ambient air temperature to spend the night, such as a small cavity or cranny in a tree. Some birds smaller than chickadees, like golden-crowned kinglets, actually roost communally, snuggling up with family members to share warmth. Chickadees generally go it alone, shoving themselves in tight spaces where the biting wind can’t reach them, and emerging in the morning for another busy day of foraging.
These nooks and crannies are not necessarily excavated by the chickadees themselves, but they are capable of that, using their bills like chisels and hammering away at soft rotting wood. You can also build a nest box for chickadees, which they will use not only in the breeding season but as a winter roost as well. Planting native trees like willow, alder and birch provides natural nesting nooks for these impressive little birds.
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