I have had the opportunity to do some illustrations for the book by Thor Hanson: "Feathers: the evolution of a natural miracle." This is the illustration of semibristle and bristle feathers.
"Bristles Bristles are essentially a stiff, barbless or lightly barbed rachis. They serve a sensory or protective function in some cases and are often found on the face, feet, or other areas of bare skin. (Those pictured here are from the face and feet of a Barn Owl.) In birds that catch insects from the air, long bristles often help direct prey towards their mouths. The vary in form -- those with enough barbs to begin resembling a standard vane are known as semibristles."
Text from "Feathers: the evolution of a natural miracle," by Thor Hanson. Copyright © 2011 by Thor Hanson. Used by permission of the author.