In the midst of a winter morning stroll with Denali, I stop and listen for neighborhood songbirds while my mixed collie sniffs the snow piled along our west Anchorage street. It’s still early enough in the day that the light is dim, but already birds are active. Their numbers include black-capped and boreal chickadees, red-breasted nuthatches, and common redpolls. All are among the smallest birds to inhabit Southcentral Alaska year round.
Bundled up in the cold, with a brisk wind adding to the chill of temperatures that hover in the mid-teens, I marvel again that these birds can survive Alaska’s winters—not only here in relatively balmy Anchorage, but also far to the north, where mid-winter temperatures regularly drop well below zero and daylight may last only an hour or two around the solstice (though the birds are helped by an extended twilight).
That such tiny creatures remain active in such extreme conditions seems one of the north’s great wonders and I applaud the birds’ winter adaptations—and toughness—while Denali and I retreat to our warm shelter, where we’ll spend the great majority of the day and night.
Because they’re two of my favorite birds and have been heavily studied by ornithologists, here I’ll share some of what I’ve been able to learn about the winter-survival strategies of black-capped chickadees and common redpolls, two of our smallest avian neighbors.
I’ll start with redpolls, which I first came to know in the 1990s after I’d begun to pay close attention to Alaska’s birds. About the same size as chickadees, with heavily streaked brown-and-white wings and back, they vaguely resemble sparrows, but have a distinctive red cap atop their head and a black chin “bib.” For much of the year they often join in flocks, sometimes numbering in the hundreds.
Observing these tiny finches in winter, and knowing that they somehow survive Interior Alaska’s extreme cold—temperatures down to minus 30 degrees or below for days or even weeks at a time—and long periods of darkness (midwinter nights last 20 hours or more), I set out to learn more about their winter strategies.
It turns out that redpolls have evolved a way to internally store the seeds they collect from willows, alders, and birches—or people’s feeders. While eating, they (like other finches) stockpile seeds in a pocket-like esophageal pouch, or crop. Once settled into a sheltered perch for the night, the redpolls then gradually digest their surplus seeds through the long hours of darkness, gaining essential calories in the process.
Exactly where redpolls spend their nights remains uncertain. Some may seek out cavities in trees while others perch together on the protected inner sections of spruce boughs. It’s also been reported that redpolls may burrow into the snow to escape the cold. For a long time, many scientists doubted this to be so, but photos of redpolls digging holes in the snowpack appear to confirm this behavior. Nowadays the reputable Cornell Lab of Ornithology reports “some redpolls roost in tunnels under the snow, where the snowpack provides insulation and stays much warmer than the night air.”
Another survival device: redpolls add about 30 percent more feathers in winter than in summer and they “fluff up” their dense plumage for added insulation, giving them the appearance of little feathered balls.
The performance of these winter coats is extraordinary. Redpolls maintain their core body temperatures at 105 degrees (Fahrenheit) even when the air temperature drops to minus 60 degrees. That’s a temperature difference of 165 degrees over a distance of a quarter to half inch, demonstrating that redpoll plumage has far greater insulative qualities than any human-made clothing.
Black–capped chickadees too have evolved remarkable winter survival strategies. Researchers have learned that the birds are “built” to withstand extreme cold and employ a variety of techniques to make it through the north’s longest, harshest season.
Like redpolls and other far-north songbirds, black caps grow additional feathers in winter, up to 30 percent more according to research I’ve found online. And they too can ruffle this abundance of feathers to better insulate their bodies and retain heat.
While their daytime core temperatures hover near 108 degrees even in subzero weather, the exposed feet of chickadees (and other northern birds) cool down to temperatures that approach freezing, a strategy that further helps the birds retain heat. Heated blood flowing from the feathered body to the feet helps to warm the blood that’s returning from the feet, minimizing heat loss while maintaining blood circulation to the bird’s chilled extremities.
To preserve their body warmth, chickadees eat prodigious amounts of food in winter. Each day they stuff themselves with enough food—primarily seeds and frozen insects—to gain an additional 10 percent or more of their weight, most of it as fatty tissue, in order to make it through the following night. Anyone who maintains a bird feeder knows their passion for black oil sunflower seeds, with their high caloric content, though as temperatures drop, black caps will also eat peanut butter and suet.
Two special adaptations help black-capped chickadees in their daily effort to pack on weight. In the fall, when food is still abundant and easy to find, they cache bits of it throughout their territories, for easy retrieval during leaner times. They’re able to do this because they have excellent memories—and brains that grow larger in winter.
Researchers have found that as autumn moves toward winter, black caps grow new cells in the hippocampus, the part of the brain that’s connected to spatial memory. This allows them to better remember where they cached food weeks earlier. It appears that those additional cells die off in the spring, when they’re no longer needed. How their brains perform this extraordinary act remains a mystery.
Black caps also depend on each other in their winter foraging. Like several other songbird species—for instance redpolls, pine grosbeaks, and bohemian waxwings—they join in flocks that increase their ability to find sources of food.
Just as amazing as their enhanced memories are the ways they survive winter’s long nights. One key is to find tree cavities that provide insulation from the frigid night air. Once inside, to save calories black caps employ what scientists call “regulated hypothermia.” They gradually lower their body temperature by 12 to 15 degrees, which in turn slows their metabolism and the rate they burn up body fat.
At the same time, the chickadees repeatedly flex their chest muscles to generate heat, which is then trapped within their puffed-up feathers. Essentially they shiver through the night. In doing so, they use up most or all of the fat reserves they gained through their prodigious eating the previous day.
After leaving their cozy roost in the morning, black caps then begin the cycle anew, joining boreal chickadees, nuthatches, redpolls, and other songbirds in the frenetic daily winter hunt for food, whether in my urban neighborhood or boreal forests in Alaska’s harsher and darker Interior region.
Anchorage nature writer Bill Sherwonit is a widely published essayist and the author of more than a dozen books, including “Living with Wildness: An Alaskan Odyssey” and “Animal Stories: Encounters with Alaska’s Wildlife.” Readers wishing to send comments or questions directly to Bill may do so at email@example.com.