The nature of feather construction
The wing feathers
The wing feathers (remiges) are used for steering. Like tail feathers, they are usually large, stiff in texture, asymmetric, have vanes that are almost entirely pennaceous, and lack afterfeathers. Wing feathers include primary, secondary, and tertiary feathers. The primary wing feathers (typically 10-11 in number) attach to the middle digit and the hand. They are asymmetrical in vane structure with the their leading and trailing margins notched. This sudden narrowing produces a series of slotted spaces between the primaries which in flight reduces air turbulence over the wing tips. Where turbulence is most extreme, the leading edge barbs are broadened and stiffened. These barbs are referred to in fly tying parlance as biots. The secondary wing feathers (anywhere from 9 to 40 in number and up to six inches wide by six feet in length) attach to the ulna of the forearm. The tertiary wing feathers attach to the humerus. There may also be a group of 3 or 4 feathers attached to the bones of the thumb forming a bastard wing (alula.) These feathers lie flat during normal flight, but extend out when flying slowly to prevent stalling.
Wing feathers may be uniquely developed for specific purposes. For example waterfowl wing feathers are designed to be water repellent. This is accomplished by modifications in the structure and position of the barbules such that a surface through which water cannot enter is created. They are so unique that a specific name is applied to this type of barbule/barbicel structure; flexules. The owl differs dramatically in having soft overlays of barbules on the surface of the feathers that keep it silent in flight.
The bases of the wing feathers as well as the upper and lower surface of the remainder of the wing are covered by several rows of small, flattened wing coverts (tectrices.) The largest wing coverts are adjacent the wing feathers digressing in size toward the wings leading edge. The vane is principally pennaceous and designed to supply an air-tight surface to the wing. The upper wing coverts, like contour feathers, are convex. Under wing coverts are concave, which fits them up into the underside curve of the wing. (This is an important consideration for the fly tier. For example in Frederic Tolfrey's Jones's Guide to Norway a component of "The Major's" wing calls for an overlay of "two snipe feathers." These are under wing coverts on the snipe, and thus are concave. Their natural shape forces the fly tier to carefully select a pair that will produce little or no outward curve when placed over the wing.) Those coverts on the leading edge of the wing initially extend vertically and then bend backward over the wing at an acute right angle creating a camber or upward curve.