Every spring, a couple of hundred species and millions of individual birds return to Alaska, taking advantage of long daylight hours and abundant food resources to breed and raise their young. Amazing as some of the migration flights are – the arctic tern, for example, comes from the opposite end of the earth, a distance of 10,000 miles or more – this feat is not nearly as astonishing as the adaptations developed by birds who reside in Alaska year-round.
There are a number of species that spend the winter in Alaska, perhaps a dozen that are commonly seen by avid birdwatcher and casual observer alike. None of them are particularly flamboyant or rare or eye-catching. Most are species that almost everyone has heard of: the common raven, the black-billed magpie, a couple of species of woodpeckers. . . and the ubiquitous chickadee.
Three species of chickadees inhabit Alaska year-round: the black-capped chickadee, the boreal chickadee, and the chestnut-backed chickadee. Of these, the black-capped is the most common. It is also well known to winter birdwatchers around the country, not just in Alaska. But how do these tiny residents survive the subzero temperatures and long hours of darkness?
These tiny birds have developed several adaptations to survive through the winter. Though they feed only during the daylight hours – definitely a disadvantage during the four and five hour days of December – they eat voraciously. Unlike other small winter residents, chickadees do not have a crop in which to store food for digestion at night. Instead, they increase their body weight by approximately ten percent during the day, and then shiver that extra fat away to stay warm all night. This is the equivalent of a 165 pound individual gaining fifteen pounds during the day and shedding it all during the cold night so that she is back to 165 pounds in the morning (don’t you wish you could do that?!).
Another way chickadees survive the long, cold nights is by roosting in sheltered, tight places. Researchers in Fairbanks have discovered that chickadees head for holes in birch trees at night, sometimes squeezing through gaps as small as a quarter. Once inside, they fluff up their feathers and lower their body temperature by fifteen degrees to conserve energy, depending on all that fat they stored up during the day to keep them warm. Unlike other bird species, chickadees prefer to roost alone at night, rather than snuggling with members of a flock to generate warmth.
Chickadees also cache food during the fall months, stashing seeds and dead insects in cracks and crevices throughout their half-mile range. The area of the brain associated with memory – the hippocampus – expands during this time of caching, then shrinks in spring when the need to find cached food diminishes. Not surprisingly, chickadees who live in harsher climates (Fairbanks vs. Denver, for example), are better both at hiding and finding cached food. Individuals who are unable to cache an adequate food supply and locate it in times of need are less likely to survive a tough winter. Finally, feathers are, of course, great insulation. Chickadees can fluff out their feathers to trap an inch of warmth around their bodies. Underneath those feathers, invisible to us, their chest muscles are flexing repeatedly to generate warmth.
When I watch the chickadees at my bird feeder, I’m always amazed by their ability to withstand temperatures of 20 or 30 below, going about business as usual. Their resilience and the adaptations they’ve developed to survive Alaska’s cold winters are inspirational.
