Down has a three-dimensional structure, and high-quality down jackets are generally very fluffy. Therefore, for the same weight, down can trap more air than other materials. The numerous fine down fibers intertwine to hold the air in place, forming an insulating protective layer, thus achieving excellent warmth retention.
Regulations stipulate that the nominal down content of down clothing must not be less than 50%, and the higher the down content, the stronger the warmth retention. When purchasing down winter clothing, you can judge the quality by observing the loft.
Birds have very full down layers. In the cold winter, they can "go naked" by using their feathers to keep the air still, insulating their own heat from the outside environment for safe wintering.