I have had the opportunity to do some illustrations for the recently published book by Thor Hanson: "Feathers: the evolution of a natural miracle." This is the illustration of a down feather.
"Down Feather A classic down feather has no rachis. Its barbs emerge from the rim of the calamus in a loose, springy tuft that gives them their remarkable insulative qualities. Practically speaking, the term down is often applied to any insulative feather, and there are a wide variety of downlike plumes with short or partial rachises. Many down feathers have elongated barbules, adding loft and increasing their ability to trap and hold air."
Text from "Feathers: the evolution of a natural miracle," by Thor Hanson. Copyright © 2011 by Thor Hanson. Used by permission of the author.