Fill Power is the number of cubic inches one-ounce of down will fill. The more cubic inches; the more fill power; the more fill power the greater the insulating property of the down. What is down? Down is the soft underplumage of geese and ducks. Down garments and bedding are warm because down is a very efficient natural insulator. It traps air in thousands of tiny air pockets. The finest quality of all down is Eiderdown. The Eider duck is a protected species, so its down is collected by hand from the nest without disturbing the bird.