As temperatures dipped into the negative numbers last week, I started wondering—how do wild birds, some no bigger than my fist, manage to stay warm in such frigid conditions? Of course, some bird avoid the problem by migrating, but plenty of birds winter right here in Colorado. I already knew that birds eat more when the weather is cold; my need to constantly refill the bird feeder is proof enough. The suet feeder, with all that high-calorie fat, empties even faster. But could a higher metabolism be enough to carry such seemingly fragile puffballs through a Colorado winter? I decided to find out.