The term eiderdown is actually a rather interesting one, as it’s a good example of a loanword from Old Norse. The original Old Norse dunna referred to a wild duck, with other terms like dúnn, dúnbeðr and dúnkoddi referring to down blankets, beds, and pillows. Presumably, settlers and traders brought these over to the Anglo Saxons, who realized how nice they were and adopted the name.
As for eider ducks themselves, they’re a species of duck that lives in the Northern Hemisphere. Iceland has a population of them, and they’re actually one of the largest ducks in Europe. Much of the down sold in jackets, sleeping bags, and pillows is not from wild eider ducks, but instead farm-raised from geese and other ducks species.
Eider ducks will use their down, a special kind of soft feather, to line their nests. Historically, eiderdown harvesting could harm duck populations, but sustainable down harvesting can also be practised after the ducklings leave the nest. Because of the method of harvesting real eiderdown, a pure eiderdown sæng (Icelandic duvet) or pillow is mostly a rarity these days, and if you do manage to find one, expect to pay a premium price for it.
