Down clusters lie closer to the body of the bird than feathers. Down has no quill and is therefore lighter, warmer and softer.
Down lies closest to the bird's body and acts as an insulating layer. Feathers, which lie further from the body, do not play this role to the same extent. Feather and down duvets, therefore, need to be heavier than pure down duvets of the same tog.
Goose down clusters are larger than those of duck, achieving greater warmth for the same weight of filling.