Black-capped Chickadee: Hypothermia & Neuroplasticity

Reasons these birds are able to brave the cold.

Let’s focus on Black-capped Chickadees. Don’t roll your eyes, chickadees are special. I restocked my birdseed to gain the privilege of watching these birds each day with my morning coffee. There are few joys akin to hearing an increase of calls and song from chickadees as I go out to fill the feeders.

The Black-capped is North America’s most widespread species of chickadee. Their range map is expansive. Much of the northern United States, much of Canada, and a good chunk of Alaska. Just a northwest smidge of California—but that state is the jurisdiction of other species: Mountain and Chestnut-backed Chickadees.

Where I bird, Black-capped are commonly seen and very well-known. If you see a chickadee and ask a non-birder what it is, chances are they’ll know. Your average joe might not be so nit-picky as to specify a Black-capped over a Carolina, but they’ll likely know it’s a chickadee.

I have upcoming birding plans—or not-birding plans that’ve turned into birding plans. What started out as “There’ll be time that day to bird a little nearby” has turned into “That’s a little over an hour away! I should go a couple times.” There’s quite a rare woodpecker not uncommonly seen at a National Wildlife Refuge. Red-cockaded Woodpecker. An endangered habitat specialist “strongly tied to old-growth pine forests that burn frequently.”

In addition to the woodpecker, I scoped out Merlin/eBird/online checklists to learn of other species I might encounter. White-eyed Vireo, Red-headed Woodpecker, the Pileated is common. Loggerhead Shrike, Prothonotary and Hooded Warblers, plenty more. Despite how ecstatic I’d be to see the woodpecker, I am also pleased at the prospect of adding Carolina Chickadee to my Big Year list. I love Black-capped, but it’ll be cool to log two species of chickadee.

Neil Barker wrote a great essay—then another—about paying a “seed tax” to the birds on his hikes. Black-capped Chickadees aren’t shy about accepting his offerings. A tax to pay for the privilege of watching the birds, the way some drives demand a toll.

These birds have adaptations allowing them to spend winters in the cold. I’d been assigned to write about ways a bird I saw on a recent outing had evolved to deal with climate. None of the birds I’d recently seen felt like good candidates—though now many come to mind. Most outings I see a chickadee. Did some research on Black-capped Chickadees. As I’ve said a thousand times before, seeing a bird regularly should not make that bird boring. It’s an invitation to know it well. There is a lot to know about Black-capped Chickadees.

A chickadee has a high metabolism. The birds typically stay over 100°F. A recent episode of the BirdNote Daily explains birds are endothermic and their bodies “maintain a constant temperature, around 106 degrees.” Numbers I came across for Black-capped Chickadees were 107° or 108°. They have to consume plenty to keep this metabolic rate up, eat pretty much nonstop. That aforementioned episode of BirdNote Daily also explains—not just about chickadees:

Birds are built for a high-energy lifestyle. Their hearts pump more oxygen-rich blood per minute than those of mammals…An active hummingbird’s heart pumps at twelve hundred beats per minute; a flying pigeon’s heart beats at 600. But a human athlete during exercise builds up a heart rate to around only 150 beats, a mere fraction of the hummingbird’s heart rate. 

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Because Black-capped Chickadees are small birds—only weighing 10-12 grams, or about as much as a AAA battery—they have a high surface-area-to-volume ratio. They lower this ratio by fluffing feathers, which saves heat. 

Feathers are massively helpful in keeping these birds warm. They have a half-inch layer which is protective when air temperatures drop. As pointed out in one article, a chickadee has “approximately 1,100 feathers during summer, but [this] increases to over 2,500 in winter. That is about a 45 percent increase in feather density.” Down captures heat next to the skin. Outer feathers are coated with oil and interlock to combat water. With such well-designed feathers, it makes sense most of a Black-capped Chickadee’s heat is lost near the eyes and bill—where feathers are not.

A chickadee will eat 60% of its bodyweight in a day; 6 grams for a 10 gram bird, 7.2 if it’s 12. Per the National Park Service, “One Fairbanks researcher often refers to these small songbirds as ‘barracudas with wings’. Black-capped chickadees are constantly searching for food and when they find it they seize it with powerful vise-like bills.”

One study weighed chickadees in the morning and found little body fat; the same birds would have ample fat by afternoon. Black-capped Chickadees tack on as much fat as they can during the day, reaching about 7%. At night, they burn it to keep body temperature high. Body fat gets down to 3% by morning.

Fascinatingly, chickadees can also enter regulated hypothermia to lower body temperature by up to 15°F. At freezing temperatures, this lower body temperature permits a bird to conserve a percentage of its hourly metabolic expenditure. Their temperature gets no lower than 86°. According to an article in The Journal of Comparative Physiology, a chickadee exposed to 0°C (32°F) displayed a 32-45% lower metabolic rate. This controlled hypothermia allows the bird to conserve more energy as air temperatures continue to drop.

Another remarkable fact is that chickadees’ brains grow about 30% in the fall when they cache food. An NPR article expresses envy at this neuroplasticity: “What Chickadees Have That I Want. Badly.”

The Black-capped Chickadee’s brain grows so it can remember where it cached food come winter. The number of neurons in the hippocampus increases; when they are no longer needed the memories are erased. The hippocampus is the hub of spatial memory. It shrinks back down in spring. To quote an article published by the Montana Natural History Center, neuroscience researchers

reviewed current ornithology studies to better understand how the expanding and shrinking chickadee hippocampus contributes to survival. They concluded that the changes in hippocampal size might help the birds to store their caches further apart in an effort to deter other hungry animals from robbing their stores. Additionally, perhaps the growth in the hippocampus is needed only for encoding the cache, or it may be that the neurogenesis occurs for both encoding and retrieving the birds’ harvest.

Goes without saying, but I am thankful for chickadees, therefore am thankful for the adaptations that help them survive—particularly when the mercury drops. Remarkable neuroplasticity, their regulated hypothermia, or simply their feathers. It’s nice on a slow birding day to hear a chickadee and know getting outside wasn’t all for naught.

As I will continue to say, a Black-capped Chickadee’s commonness is an invitation to know it well. It’s up to us to do so. I might get to see them each day at the feeder, but it’s their remarkableness that allows me to see one and think: That bird’s brain size fluctuates commensurate with its survival needs! And it can go into controlled hypothermia. A remarkable little creature. They offer no shortage of knowledge to gain.