When the weather turns cold, it is not unusual to see chipping sparrows, northern mockingbirds, and other feathered backyard residents look much larger than normal. These birds have not suddenly put on an excessive amount of weight. To the contrary, they are simply trying to keep warm.
Birds have a number of ways to keep from freezing when temperatures plummet. One way is to fluff out their feathers. This works because when a bird fluffs its feathers, air pockets are created between a bird’s skin and the feathers. The heat generated by the bird’s body is trapped in these air pockets. In turn, the combination of the feathers and air pockets acts as an excellent insulation. In fact, feathers provide more insulation than hair.
If you wear a down-filled coat in winter, you can attest to the ability of feathers to ward of the cold.
When we go outside to brave the cold we have to put on clothing to keep warm. Birds, on the other hand, are able to protect themselves with additional insulation by simply fluffing out their feathers at will. This is accomplished thank to an amazing network of tiny muscles that move each individual feather into position to create a fluffy appearance.
Interestingly, small birds such as chickadees have greater heat loss problems than larger birds. This is because larger birds possess less surface area over which heat can be lost in comparison to their volume than smaller birds.
American goldfinches and some other small birds actually grow more feathers when winter looms on the horizon. Since the additional feathers offer added insulation, this problem is partially alleviated.
Nature is indeed amazing!