On this Moment of Science, we discuss one of the reasons birds fluff up their feathers.
If you've ever seen birds fluffing up their feathers, it's most likely because they are cold. Birds have different kinds of feathers. The large ones we see when a bird stretches its wings are "flight feathers," but they also have fluffy ones called "down feathers." Some birds, such as goldfinches and redpolls, grow more of these in cold weather. These are used to help keep the bird warm.
Birds also utilizes the air in order to keep warm. You see, air is an excellent insulator. That means that heat doesn't pass well through it. When a bird puffs up its down feathers, it's trapping air in-between them. This has the result of surrounding the bird's body with a layer of air that acts as an insulating shield. That way the bird's own body heat won't escape as easily. It's the same principle that makes a big, thick winter jacket keep you warmer than a thin, tight shirt. So when birds get cold . . . they fluff up the down.