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 and featherbeds), 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. The down feather is considered to be the most "straightforward" 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. Body down is a layer of small, fluffy feathers that lie underneath the outer contour feathers on a bird's body. Powder down Powder down, or pulviplumes, is a special type of down that occurs in a few groups of apparently unrelated birds. In some species, the tips of the barbules on powder down feathers disintegrate, forming fine particles of keratin, which appear as a powder, or "feather dust", among the feathers. These feathers grow continuously and are not moulted. In other species, powder grains come from cells that surround the barbules of growing feathers. These specialized feathers are typically scattered among ordinary down feathers, though in some species, they occur in clusters. All parrots have powder down, with some species (including the mealy amazons) producing copious amounts. It is also found in tinamous and herons. The dust produced from powder down feathers is a known allergen in humans. Function The loose structure of down feathers traps air, which helps to insulate the bird against heat loss and contributes to the buoyancy of waterbirds. Species that experience annual temperature fluctuations typically have more down feathers following their autumn moult. There is some evidence that down