The down feathers of eider ducks provide insulation due to lack of barbules and ability to fluff.

References

"The down feathers are small, with a short shaft and no hooked barbules, so the barbs to not interlock but fluff out in different directions. They can also be packed into a small space and re-expand afterwards to their original fluffiness. Down is the covering of small chicks and other nestlings, and provides excellent insulation, as anyone with a down-filled quilt can testify. The down of the eider duck has become synonymous with the best type of quilt. When the female eider duck nests in the cold springtime of its home in Scandinavia, she pulls tufts of thick down from beneath the contour feathers on her breast, and uses the down to make a thick. warm lining for her nest…Down is extremely light, making it ideal for cold-climate clothing and equipment: down-filled suits and sleeping bags have revolutionized mountaineering and polar exploration." (Foy and Oxford Scientific Films 1982:109)

"One of the warmest nests in the world, the eider duck's nest is lined with soft down from the female eider duck's breast. Eider ducks nest on tundra-covered islands in the arctic, so it is not surprising that they have developed the warmest known down. When they hatch, the chicks already have their own eiderdown coats." (Foy and Oxford Scientific Films 1982:111)

Book
The grand design: Form and colour in animalsNovember 21, 1983
Sally Foy

Living System/s

Organism
King EiderSomateria spectabilisSpecies