Eiderdown is down that comes from the Eider, a large sea duck that lives on the northern coasts of Europe, Asia, and North America. The Eider's nest is built close to the sea and is lined with the eiderdown. This soft and warm lining has long been harvested for filling duvets and pillows. Although eiderdown comforters and pillows are now a rarity, eiderdown harvesting continues and is sustainable, as it can be done after the ducklings leave the nest with no harm to the birds. Eiderdown is exceptionally soft and has insulating properties far superior to any other down.
Eiderdown is such a unique plumage. Fill power cannot accurately be measured because of the way each cluster clings to the other. Typical goose and duck down tends to sit loosely in a duvet or pillow and "flow." Eiderdown tends to hold together and is generally more resilient. This clingy behavior also makes it better at trapping air and, therefore, a better insulator.