In late October of last and this year, an Arctic front blasted the Intermountain West with frigid temperatures (-7° F) and a dump of wet snow. The systems were abrupt and harsh, not only to us supposedly tough mountain folk, but the poor birds still migrating south.
While I fretted for the migrants, I was much less worried about our resident birds and the altitudinal migrants pushed downslope by the storm. These birds that stay north when the mercury plummets and the snow flies have many superb adaptations that help them survive even in the most frigid conditions.
Birds are warm-blooded, maintaining a constant deep-body temperature of about 105°F (40°C) so they can sustain their metabolic functions. But it can be a challenge for birds to maintain their inner furnace through howling snowstorms and subzero temperatures, particularly smaller birds with very little internal volume. Winter survival means facing two daily existential dilemmas: don’t freeze, and don’t starve.
In this Chorus, I thought it would be interesting to look at the many physical and behavioral adaptations of birds that help them survive the cold winter months using (mostly) my photos from many an icy winter in Idaho and Montana to illustrate each adaptation. I encourage you to add your own photos and observations in the comments!
PHYSICAL ADAPTATIONS
Feathers — Feathers are literally the first layer of defense against the cold, providing remarkable insulation. Many bird species grow extra feathers as part of a late-fall molt to give them thicker protection in winter. Larger birds (like geese and grouse) and water birds in cold climes (like the American Dipper) add a layer of insulating down feathers. A waterproof coat of oil from birds’ uropygial gland provides further weatherproofing and protection from the double-whammy threat of being cold and wet.
Legs and Feet – How do the delicate legs and feet of birds withstand frostbite? Rete mirabile – Latin for “wonderful net” is how. Birds have a counter-current heat exchange system with a fine, netlike pattern of veins and arteries that keep their core toasty while constricting blood flow to their extremities. The system minimizes heat loss, but allows enough warmth to keep the feet from freezing. And because bird circulation is fast, blood doesn’t linger in the feet long enough to freeze. It also helps that there is so little fluid in the cells of bird feet, and those feet are mostly bone and tendon with only bits of muscle and nerve tissue.
Fat Reserves – Birds work diligently in fall seeking and gorging on abundant food sources to build up fat reserves ahead of winter. In species such as chickadees and finches, more than 10% of winter body weight may be fat, keeping these small birds both insulated and fueled to survive frosty nights.
BEHAVIORAL ADAPTATIONS
I’m Not Fat, I’m Fluffy – On those extra-cold days, a bird will fluff out its feathers to create air pockets that trap heat between its body and feathers. The more trapped air, the more insulation from cold, the warmer the bird.
Tucking – Birds often stand on a single leg or crouch atop both legs to shield exposed skin from the cold. Birds will also tuck their bills into their shoulder feathers to shield it from cold and allow the inhalation of air warmed by their body heat.
Sunning – Many birds take advantage of a good solar bath on a sunny winter day. They will perch with backs to the sun, exposing the largest surface of their bodies to the heat, with feathers slightly raised to heat skin and feathers more efficiently. Birds will also spread their wings and tail to maximize their surface area exposed to the sun’s rays.
Shivering – Birds shiver to raise their metabolism and stay warm in extreme conditions. This isn’t the usual tremor that mammals use to generate heat. Bird shivering is activated by opposing muscle groups, generating contractions without all the shakiness typical when humans shiver. Bird shivering is much more efficient at conserving calories and retaining body heat, but is usually a short-term solution to generate heat.
Roosting – Both individual birds and small flocking birds use dense vegetation, tree cavities, empty birdhouses, and roost boxes to find protection from the elements at night. Birds may select roost spots that retain residual heat, such as a solar-warmed tree trunk or the eaves of a warm building. Communal roosting by a species — huddling together to share warmth — awards critical energy savings during frigid spells.
Torpor – Torpor is a state of reduced metabolism when body temperature is lowered, thereby requiring fewer calories to maintain body heat. Though not very common in winter birds, an exception is the Black-capped Chickadee which does a more moderate version of torpor called “regulated hypothermia” in which body temperature is reduced as much as 22°F to conserve energy on frigid nights. The downside of torpor is that the bird is less alert and more prone to predation. It also takes energy to come out of torpor, requiring an immediate need for food once the bird is active again.
HOW TO HELP KEEP BIRDS WARM
For birds, surviving a frigid winter is a matter of mastering the equation of energy input versus output, taking into account all the variables (predators, disease, available food and shelter, weather vagaries), and always leaving enough calories to live another day. We — as bird lovers and backyard stewards — can do a lot to give our local birds an edge over the most severe winter weather.
- Offer Quality Food – Hang feeders and select seeds, suet, nuts, peanut butter, scraps, and other items high in fat and calories to give birds plenty of energy to generate more body heat. Check out Project FeederWatch’s Food and Feeder Preferences of Common Feeder Birds for tips on attracting a diversity of birds and selecting the best feeder design and food types for backyard birds.
- Keep Feeders Full – Birds need to replenish their energy reserves before and after a long, cold night. Be sure to keep feeders full so birds know where to beeline for a high-energy buffet.
- Offer Liquid Water – Access to liquid water is critical to birds year-round, but is especially important in the deep-freeze of winter. By offering birds a drink from a heated bird bath, they will use less of their precious energy and body heat to melt ingested snow and ice.
- Provide Shelter – Evergreen shrubs and coniferous trees provide safe, protected roost sites for flocking birds to conserve and share body heat. Brush piles also offer a sheltered place to roost and forage. Putting up bird houses and roost boxes is also helpful to overwintering birds.
Thanks for all you do to keep our feathered friends warm, fed, watered, and protected, especially when the weather outside is frightful.
A friendly reminder, the 2020-2021 Season of Project FeederWatch started yesterday — Saturday, November 14 — and runs for 21 weeks to Friday, April 9.
Turn your love of feeding birds into scientific discoveries!
The floor is now open for your latest bird observations and other avian news!