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 filoplume feather.
"Filoplume Unlike most other feathers, filoplumes lack muscles within their follicles and cannot be adjusted of moved independently. Their role is sensory, providing the bird with information on the movement and condition of other feathers nearby. Clusters of filoplumes usually surround the base of each flight feather. They act like telltales on a sail, giving instant data on wind speed and feather position and helping the bird make fine adjustments during flight. In rare cases, elongated filoplumes serve a function in breeding displays, elongating so that their tufted tips emerge dramatically from the surrounding body feathers."
Text from "Feathers: the evolution of a natural miracle," by Thor Hanson. Copyright © 2011 by Thor Hanson. Used by permission of the author.