The down insulation used in good quality outdoor clothing and sleeping bags consists of tiny clusters from the plumage of ducks and geese. These clusters are made up of huge numbers of tiny microscopic "fingers" that trap air and body heat, creating a high level of insulation with very little weight or bulk. Very small amounts of the larger and heavier feathers are normally found mixed with down filling and the fewer the number of feathers the higher the quality of the down. Goose Down will have a greater insulation value per gram than Duck Down. In all other respects good quality Duck Down has the same properties
Down fillings have a superior warmth to weight ratio than most, if not all synthetic fibres and is incredibly compressible. Down is also a very comfortable filling due to its breathability allowing moisture to escape and has the ability to self-regulate its insulation level by reducing its loft when you get hot.
The down used by companies such as Rab and Montane has been treated for increased water repellency by a factor of eight. This means that, although you should still try to keep down filled sleeping bags and jackets dry, they will resist more dampness than untreated down, dry more quickly and perform better in high humidity such as when used under a waterproof outer.
Grades or quality of down: The quality of down is measured in Fill Power. The greater the volume that 1 gram of down fills the higher its rating. The higher the Fill Power the lighter the fill weight needed to be able to achieve the same insulation level.