This black capped chickadee is utilizing several strategies to keep warm in this bout of subzero weather we've been having. Some have said, and I can't remember exactly where I've heard this, that according to the laws of physics, chickadees should not be able to survive extremely cold weather. Obviously, this bird is not a law student.
One, it is eating. Luckily it is part of a flock that has found a reliable seed source (that would be me). The chicks get the bonus of what to them is an all-you-can-eat buffet, and I get the bonus of being able to watch them from my somewhat climate-controlled shelter.
Two, it is fluffed out to the max. Those down feathers, with their amazing air-pocket trapping capacities, are the equivalent of a good down comforter or parka. Goose, that is, not chickadee.
Three, it is sitting in the sunlight, in an area sheltered from the wind. Microclimate makes all the difference. I wonder if the black cap helps it to absorb more heat from the sun in its cranial area.
Four, it spent the night roosting, probably in an old woodpecker nest in a dead tree (leave 'em standing!), with a bunch of other chickadees. The bent tail feathers I see on some individuals attest to the fact that when it comes to sleepover parties, the more the merrier. The tail feathers sometimes get bent when things get crowded, but all benefit from the body heat and retention from all that chickadee down. But body heat isn't everything; I have heard chickadees can drop their internal temperature at night to save energy.
We humans have slightly different adaptive strategies. We build unnaturally shaped rectangular domiciles and fill the walls with glass fiber fluff. We burn wood inside these shelters, and even sometimes bottles of liquid propane. Sometimes both at the same time. We put on warm clothing. KIDS--REPEAT--WE PUT ON WARM CLOTHING!! Today we even took a few further adaptive measures, like putting plastic on the three porch windows that we'd been too lazy to do before. And The Hermit had to fix the propane heater, which somehow had gotten polluted by some foreign plastic object that somehow got in there. Both measures had measurable effects; I'm not freezing sitting here at the computer and the temperature at about 10 feet up in the main room topped 100. Again, microclimate is everything; on the floor under the bunk bed, a can of pop will still freeze.