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The avian wing is a marvel of engineering: lightweight but incredibly strong, stiff yet flexible, forming an airfoil that is instantly adjustable to any conditions, and when not in use folding into a thin panel and tucking neatly out of the way along the side of the bird’s body. Understanding how the wing works and where the different feather groups go as the wing spreads and folds is an important bit of fundamental bird-ID knowledge. In this column, I’ll explain how the wing fits into the body feathers at rest.
The wing is composed of specialized flight feathers and coverts, adapted to be stiff and flat to stand up to the rigors of flight and to create a good aerodynamic surface. They are not so good for insulation. This means that when the wing is folded along the side of the body, other feathers must keep the bird warm.
In my column in the August 2017 issue (“Avian air conditioning”), I described how songbirds use wing position to regulate their body temperature. In extreme cold, the flank feathers fluff out from below, along with the scapulars from above, so that these body feathers cover most of the wing and insulate the body.
Birds that spend a lot of time swimming face the added challenge of keeping water away from their body. Ducks and geese take wing stowage to the next level — the wing is almost entirely hidden all of the time. On a swimming duck, the flank feathers wrap up around the sides to cover most of the wing, and the scapulars spread down to meet the flank feathers. The only parts of the wing left exposed are the largest feathers visible toward the rear of the bird, generally a couple of large tertials (the innermost wing feathers) and the tips of the longest primaries. Everything else is hidden underneath the waterproof shell of flank and scapular feathers. In a way, the duck is riding in a “boat” of feathers: The long flank feathers extend above the waterline to form the gunwale. The scapulars act as part of a canopy, and the folded wing simply tucks down inside it all.
In birding, knowing what you can’t see is often helpful, and on a swimming duck, keep in mind that you will see very little of the wings.
This article from David Sibley’s “ID Toolkit” column appeared in the May/June 2018 issue of BirdWatching.