The female common eider pulls down from her own breast to insulate her eggs, producing a mat of exceptionally fine, tangled filaments that trap air far better than most feathers. This is the material sold as eiderdown.
In Iceland, the source of most of the world's eiderdown, farmers protect nesting colonies from predators and disturbance, then take down from nests, typically gathering after the young have left or in small amounts during incubation with replacement material added. The annual global harvest is on the order of a few thousand kilograms, and a single nest yields only a small quantity, which is why eiderdown remains among the most expensive natural fibres.
The common misconception is that the down is plucked from live or killed birds. It is not, in the regulated Icelandic system. What is genuinely uncertain is the species' trajectory: eider numbers have fallen in parts of the Baltic and North America, and the causes, which likely include changing prey, disease and predation, are not settled.