Looking like balls of fluffy cotton, this group of swallows huddle together during a winter blizzard.
By fluffing both their downy and semiplume feathers, they are able to trap pockets of air close to their body. How much body heat they keep can adjusted by arranging their feathers to trap more or less air. At the same time, the stronger and ridged contour feathers shield birds from wind. The tough material they are made from, beta-keratin is both water and wear resistant.
Source: ASU School of Life Sciences