The best down is derived from large, mature birds, but generally, goose down is better than duck down. On the other hand, down from a mature duck is better than down from an immature goose.
Down from younger birds has poor lofting ability and collapses in a relatively short time, because the fibers are fragile. A duvet made with a mature down will be warmer, lighter and puffier and last longer than a duvet made with immature down.
Each ounce of high quality down contains around 2 million fluffy filaments that interconnect and overlap to form a protective layer of air that keeps warmth in and cold out. These tiny pockets of air trapped by the down fibers create the insulation factor. The more mature the down, the greater number of air pockets there are for more efficient insulating. Because of its resilience, you can flatten out the duvet, and all it takes is a good shake to fluff it back up.
Down provides approximately three times more warmth per ounce than synthetics. Less down is required by weight than other fillings, which makes for a lighter but warmer product. Synthetics mat and clump together over time, leaving empty cold spots, while down continually refluffs, and molds itself to the body. Down also has the wonderful ability to breathe and eliminate perspiration.
If cared for properly a high quality down will last for years and years. The Down Association of Canada has recovered down duvets with fillings which were still in very good condition and are several decades old.
Eiderdown comes from the Eider duck which is a protected migratory species and is considered to be the best quality down, and the most expensive. This down is unique as it clings to itself, attaining superior insulating power. Down is collected by hand from nests without disturbing the birds and only about 15-20 grams of down is allowed to be recovered from a nest. The colour of Eiderdown can range from brown to almost black.
You’ll find a wide range of cost for down products. An inexpensive product will be filled with immature down, which will provide little warmth, and will most likely collapse after relatively little use. Shortcuts in the cleaning and processing will have been taken, and this will often lead to odor problems. The Down Association of Canada has also found that a substantial number of inexpensive offshore products have contained high percentages of feathers, rather than down as promised on the label. Workmanship and design are generally inferior on cheap products. Overall, a "bargain" is usually a waste of money. A quality product will cost more, but it will be warm, cozy and last for many years.