Editing Down feather (section)

==Description and etymology==
[[File:Chick.jpg|thumb|right|Like many [[precocial]] hatchlings, domestic [[chicken]]s are already covered with a coat of downy feathers when they hatch.]]

The word ''down'' comes from the [[Old Norse]] word ''dúnn'', which had the same meaning as its modern equivalent.<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/down |title=<sup>7</sup>down (noun) |publisher=Merriam-Webster Dictionary |access-date=2009-05-29}}</ref> The down feather is considered to be the most "straightforward" of all feather types.<ref>{{cite book |title=The Integumen: A Textbook of Skin Biology |first=Richard Ian Campbell |last=Spearman |page=[https://archive.org/details/integumenttextbo00spea/page/97 97] |year=1973 |isbn=978-0-521-20048-6 |publisher=Cambridge University Press |url=https://archive.org/details/integumenttextbo00spea/page/97 }}</ref> It has a short or vestigial [[rachis]] (shaft), few [[barb (feather)|barb]]s, and [[barbule]]s that lack hooks.<ref name=HBW1>{{cite encyclopedia |title=Class Aves (Birds) |encyclopedia=Handbook of Birds of the World, Vol. 1: Ostrich to Ducks |location=Barcelona |publisher=Lynx Edicions |first=Eduardo |last=de Juana |editor=del Hoyo, Josep |page=39 }}</ref> 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 [[pennaceous feather|contour feathers]] when they hatch.<ref name="Campbell1985">{{cite book |last1=Campbell |first1=Bruce |last2=Lack |first2=Elizabeth |title=A Dictionary of Birds |date=1985 |publisher=T and A D Poyser |location=Carlton, England |isbn=978-0-85661-039-4}}</ref>{{RP|470}} '''Body down''' is a layer of small, fluffy [[feather]]s that lie underneath the outer contour feathers on a bird's body.<ref name=Sibley>{{cite encyclopedia |title=Flight, Form, and Function |encyclopedia=The Sibley Guide to Bird Life & Behaviour |page=17 |year=2001 |publisher=Christopher Helm |location=London |isbn=978-0-7136-6250-4 |first1=Chris |last1=Elphick | first2=John B. Jr. | last2=Dunning |first3=Rich |last3=Cech |first4=Margaret |last4=Rubega |editor=Sibley, David |editor2=Elphick, Chris |editor3=Dunning Jr., John B.}}</ref>

===Powder down===
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[[File:Bird_Imprint_on_Window_0069.jpg|thumb|300px|alt=Faint white outline of bird's outstetched wings and body on windowpane|"Feather dust" left on a window after a birdstrike]]

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.<ref>{{cite book |title=Home Study Course in Bird Biology, second edition |publisher=Cornell Laboratory of Ornithology |year=2003 |editor1-last=Podulka |editor1-first=Sandy |editor2-last=Rohrbaugh |editor2-first=Ronald W. |editor3-last=Bonney |editor3-first=Rick |page=55 (Glossary)}}</ref> In other species, powder grains come from cells that surround the barbules of growing feathers.<ref name="Campbell1985" />{{RP|208}} These specialized feathers are typically scattered among ordinary down feathers, though in some species, they occur in clusters.<ref name=HBW1/> All [[parrot]]s have powder down, with some species (including the [[mealy amazon]]s) producing copious amounts.<ref>{{cite book |title=Parrots: A Guide to the Parrots of the World |first1=Tony |last1=Juniper |first2=Mike |last2=Parr |year=2003 |publisher=Christopher Helm |location=London |isbn=978-0713669336 |page=17}}</ref> It is also found in [[heron]]s and [[tinamou]]s.<ref name=HBW1/> The dust produced from powder down feathers is a known [[allergen]] in humans.<ref>{{cite book |title=Allergic to Pets?: The Breakthrough Guide to Living with the Animals You Love |first=Shirlee |last=Kalstone |publisher=Bantam Dell |location=New York, NY |year=2006 |pages=[https://archive.org/details/isbn_9780553383676/page/34 34–35] |isbn=978-0-553-38367-6 |url=https://archive.org/details/isbn_9780553383676/page/34 }}</ref>