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down feather
The down of birds is a layer of fine feathers found under the tougher exterior feathers. Very young birds are clad only in down. Powder down is a specialized type of down found only in a few groups of birds. Down is a fine thermal insulator and padding, used in goods such as jackets, bedding (duvets), pillows and sleeping bags. The discovery of feathers trapped in ancient amber suggests that some species of dinosaur may have possessed down-like feathers.
==Description and etymology==
The word ''down'' comes from the Old Norse word ''dúnn'', which had the same meaning as its modern equivalent.〔(【引用サイトリンク】title=7down (noun) )〕 The down feather is considered to be the "simplest" of all feather types. It has a short or vestigial rachis (shaft), few barbs, and barbules that lack hooks. There are three types of down: natal down, body down and powder down. Natal down is the layer of down feathers that cover most birds at some point in their early development. Precocial nestlings are already covered with a layer of down when they hatch, while altricial nestlings develop their down layer within days or weeks of hatching. Megapode hatchlings are the sole exception; they are already covered with contour feathers when they hatch.〔Campbell & Lack, p. 470〕 Body down is a layer of small, fluffy feathers that lie underneath the outer contour feathers on a bird's body.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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