It’s waterfowl down feather used for the cotton batting of umo-futon (feather-comforter). Down is fluffy feathers grown around the chest of water birds and it looks like dandelion puffs. Down contains a lot of air and aerial, and holds warmth. Only small amounts of down are obtained from one bird, so an umo-buton (feather-comforter) with lots of down feathers in it becomes so expensive.