Why do people that have a duvet filled with feathers shake it on cold winter night?

People shake their feather-filled duvets on cold winter nights to redistribute the feathers within the duvet. Over time, the feathers can clump together, creating uneven distribution of warmth. By shaking the duvet, the feathers are fluffed up and evenly spread out, ensuring better insulation and warmth throughout the night. This practice also helps maintain the overall quality and longevity of the duvet.