Feathers keep birds warm, help them hide or show off, and some let them fly. Made from keratin, they have a hollow shaft  in the middle, which branch out into thinner shafts called barbs. From each of these barbs come many tinier branches called barbules.  To keep birds warm and dry in cold or wet weather birds also have fluffy down feathers, hidden beneath a tougher layer of contour feathers that cover most of the bird's body - these are fluffy at the bottom, smoother at the top. I am fascinated by how well this waterproofing and wind proofing works for the many different types of birds there are.