It’s a feather of water birds used for the fillings of umo-butons. The feather consists of a central rachis with barbs and barbules. A small-sized feather is called a small feather. A futon filled with at least 50% feathers or more is called umo-buton. However, the feather-blended fillings tend to be a little hard to the touch because of the rachises and enough hard to retain their shapes, so they are suitable for pillows or cushions.