One November, several years ago, I went snowshoeing in the woods surrounding my home, a part of the Black Forest that grows along the divide separating the Arkansas River and South Platte River watersheds in Colorado. A big winter storm blanketed the forest with two feet of snow on top of an eight-inch base left from October. The morning was surprisingly cold, considering the snow was still falling. I breathed heavily as I trudged up a hill on my snowshoes. As I approached the hill’s crest, I stopped for a moment to catch my breath. Finally, I began to listen. No birds. The forest was quiet but for the wind slipping through the trees.

I continued snowshoeing. Eventually, I came upon a small, round gray bird. It had a narrow tail, a large head with white cheeks, a black cap, a black throat, and white eyebrows –  a mountain chickadee. The bird was perched in the crook of two large pine branches, its feathers all puffed out, presumably for warmth. I noticed it was shivering uncontrollably. I felt so sorry for the poor bird that I briefly considered plucking it from its perch and sticking it in my parka to warm it. But, of course, I knew better than to interfere with the course of natural events. I figured out later why I was right not to intervene.

Chickadees are widespread across North America, including in the Rocky Mountains. The more common black-capped chickadee makes its home at lower elevations in the large cottonwood trees and willows along rivers and creeks. Their less familiar cousins, the mountain chickadees, inhabit the higher montane forests of ponderosa pines, spruce, and aspen like those near my home.

While I frequently saw chickadees, I rarely paid them much notice, as they were quite common. Seeing a chickadee did not bring the same delight as spotting the first arrivals of spring’s colorful birds, such as bluebirds, warblers, and tanagers. But I began to wonder, are these common little birds really that unremarkable?

With the coming of winter, the weather grows much colder and food becomes scarce for many animals. They have three possible survival strategies in winter: hibernate, migrate, or tolerate, or sometimes a combination of these. Cold-blooded animals like reptiles and amphibians descend into a form of hibernation called brumation, where their body temperature and metabolism decline and most bodily functions cease as ambient temperature falls.

On the other hand, warm-blooded mammals and birds have thermoregulating processes that generate body heat internally, maintaining a set body temperature despite the variation in the outside temperature. This allows warm-blooded animals to remain active and tolerate cold weather, but it requires additional energy to generate the extra body heat. While this works up to a point, with extreme exposure, the animal’s muscles begin to shiver to generate extra heat. Past that point, uncontrolled hypothermia sets in, and the body’s temperature drops, which could be lethal.

Birds don’t hibernate, but their ability to fly allows many to migrate to warmer climes. In the northern hemisphere, many songbirds will head north in springtime to the feeding and breeding grounds of the arctic tundra, boreal forests, northern plains, and Rocky Mountains. By summer’s end, they’ll fly south again to avoid the harsh rigors of winter. They trade off the stresses of enduring winter for the stresses of long-distance migration, often traveling hundreds or thousands of miles.

Since resident songbirds like the chickadees don’t migrate, they must somehow deal with the two main challenges of winter – heat loss from intense cold and food scarcity. A chickadee only weighs about 11 grams, or roughly the weight of a ballpoint pen. At first, it might seem unlikely that such a small bird with a high metabolism could find enough energy to stay warm in winter. Indeed, a chickadee has a rather slim margin for survival. Yet, somehow, many of them manage to survive in temperatures even lower than 40 degrees below zero at night.

A chickadee’s behavior isn’t as random as it might appear. In fall and winter, it’s all about food. A chickadee must be highly focused on carefully maintaining a positive energy balance – taking in more calories in food than it expends on essential activities such as acquiring food, caching food, and generating body heat. Metabolic requirements are different in winter than in summer, so animals must acclimate to cold before winter adaptations are expressed. The decreasing daylight and normal decline in temperature during autumn induces cold acclimation by transforming a bird’s energy metabolism. However, while rare, if a severe cold snap occurs early before acclimation takes place, many birds might die.  

One autumn morning, while I sat quite still in my usual spot next to a grove of pine and aspen trees, I spotted a chickadee and watched its antics with interest. It flew to a birdfeeder, collected a sunflower seed, and flew back to a nearby aspen tree. As it hung upside down from a twig, it pecked at the seed, hammering it into a crevice in the bark. I realized that instead of collecting seeds to eat, the bird was busy storing seeds in a cache for later. 

Many birds that endure winter address the scarcity of food by storing caches in summer and fall. As it turns out, chickadees and a few other birds are referred to as scatter-hoarders, collecting seeds and bugs to hide in thousands of places scattered over a large area. Scattering food caches, while taking more energy, mitigates the risk of having all their precious food pilfered by other birds or squirrels. Then, by winter, chickadees spend most of their short daylight hours stuffing themselves with seeds and frozen insects they cached earlier. They eat so much that they gain an additional 10% of their body weight in fat each day, which they then burn off each night.

They will roost at night in dense foliage or small holes or crevices in trees tucked out of the wind. They’ll puff out their feathers to trap warmth close to their body. According to the Cornell Laboratory of Ornithology, black-capped chickadees use regulated hypothermia to reduce their body temperature by up to 22°F. This helps use precious fat reserves more efficiently. This regulated torpor is a form of hibernation, but less extreme and only occurs during the night, and hence is called nocturnal hypothermia. Non-shivering heat produced by burning brown fat combined with heat from muscle shivering maintain the chickadees’ body temperature during long, frigid nights. As the margin for survival is slim, it could be lethal if something disturbs a chickadee, forcing it from its night roost. I think I made the right decision years earlier to not disturb the shivering chickadee. In this case, the bird was shivering to raise its temperature back to its normal 108 degrees and to regain full alertness. Had I tried to “rescue” the chickadee while its body temperature was still low, the stress probably would have sent it into shock.

I wondered, how does the chickadee remember precisely where every seed is hidden? The chickadee has twice the brain size of many other birds relative to body size and has an incredible memory. As a chickadee stores a piece of food, it remembers the location by adding new neurons and making new connections. With each new memory, its brain grows larger. In the end it has built a mental spatial map of all the locations where food is cached. As cached food is depleted and new food items are cached, the original neurons are replaced by new ones forming new memories. Eventually, the chickadee’s brain shrinks as cached food is used up and new food sources like insects become abundant again in spring.

Research conducted by Vladimir Pravosudov at the University of Nevada demonstrated that the brains of chickadees increase in size the further north or the higher elevation they live. These findings correlate with prolonged and colder winters that require birds to store more food and hence grow bigger brains.

I am at an age now when I find myself increasingly misplacing my car keys, eyeglasses, or cell phone. I am rather impressed with and a bit jealous of the mental feats that a brain, only twice the size of a green pea, is capable of.

I recently went for a hike one winter afternoon while I pondered what I had learned over the years about chickadees, including their remarkable adaptations to winter. I’ve since learned they are one of the most intelligent birds, as scatter-hoarders tend to be, with very complex songs. Other birds listened closely to the chickadees, who are often the first to find new food sources or spot the arrival of predators. 

After a while, I became chilled and decided to head home. I grabbed an arm full of firewood near the house before heading inside. I opened the wood stove door and poked at the coals until they glowed. The kindling caught, and flames grew bright. I found comfort in a cup of hot tea while sitting near the fire. My thoughts turned back to chickadees. I once took them for granted, but no longer, as chickadees taught me that if you study closely those things which appear commonplace in nature, you often find something quite extraordinary. As my mind began to drift off, I felt lucky I didn’t need to shiver to fall asleep or to wake myself up in the morning.