Birds fluff up their feathers for various reasons, including thermoregulation, waterproofing, and communication.
Fluffing up their feathers helps birds trap air close to their bodies, creating an insulating layer that helps them retain heat. This is especially important for small birds, which have a higher surface area to volume ratio and lose heat more easily than larger birds. Fluffed-up feathers also help to keep water out, making them an important adaptation for birds that live in wet environments.