The National Wildlife Federation's newsletter this morning included a nice article on birds in winter and how these charming little animals stay warm. My feeders are full, the snow continues to fall, and I've refilled the bird bath with warm water twice this morning. From the desk of Melissa Mayntz, NWF:
Birds’ feathers provide remarkable insulation, and many species grow an extra layer of down as part of a late-fall molt. Feathers are aligned to create tiny air pockets, and their outer layer is coated with waterproofing oil produced by a gland at the tail’s base and distributed when a bird preens.
The key is layers of trapped air contained between overlapping feathers that, when warmed by body heat, act as a cocoon of warmth,” says biologist Gavin Bieber of Wings Birding Tours Worldwide. “Think how a cushy down jacket with an outer waterproofing layer works for us.” As for featherless legs and feet, they’re covered with scales that minimize heat loss.
When fall food is plentiful, birds gorge to build up insulating fat, which also provides fuel to conserve body heat. Some species switch to higher-fat diets in winter. On sunny days, birds take advantage of solar radiation, turning their backs to the sun to allow their largest surface areas to soak up the rays. Under clouds, they may shiver, which burns calories but increases body temperature.
Roosting is another behavioral adaptation. “Small flocking birds such as bushtits, chickadees, nuthatches and titmice manage cold northern winters by roosting in groups in tight cavities,” Bieber says.
The most extreme survival strategy is torpor: a state of lower metabolism and body temperature that conserves energy. Hummingbirds regularly undergo torpor while swifts, doves and chickadees do so in extreme conditions. The common poorwill can enter a torpor so deep it effectively hibernates—the only bird species known to hibernate through winter. Compared with that, our coats and hats may seem like primitive adaptations indeed."