The wing is one of the most remarkable parts of a bird. When extended in flight, it serves as a lightweight and efficient airfoil. But when birds are at rest, they fold their wings and hold them tight against the sides of their bodies. The two positions look entirely different. Learning just a few key points will help you understand and reconcile the varying shapes and patterns.
The wings of all birds share the same basic structure, consisting of flight feathers and wing coverts.
Two types of feathers make up the flight feathers: primaries and secondaries. The primaries are the long feathers that form the wingtip. They attach to the avian skeleton’s “hand” bones. The secondaries are the big feathers that form the inner part of the wing and are attached to the “arm” bones. Each of the flight feathers overlaps the outer feather next to it and slides under the next feather closer to the body.
As a result, when the wing is folded, the feathers form a tight stack; the outermost primary goes on the bottom, while the innermost secondary goes on top. The bases of the flight feathers are protected by rows of wing coverts, becoming smaller right up to the leading edge of the wing.
Wings come in a tremendous array of sizes and shapes. Aerial species like Barn Swallow are at one extreme; terrestrial species like House Wren are at the other. But the structure of the wing is the same from bird to bird. The primary feathers always form the point at the tip of the folded wing. They are simply much longer and more prominent in swallows, shorter and almost completely hidden by the secondaries in wrens.
The wing shape of most songbirds, including American Goldfinch, falls somewhere between the extremes of swallows and wrens. Subtle differences between species are linked to differences in habits and behaviors. Learning the common structure, and looking for similarities between species, can help you appreciate the differences.
This article from David Sibley’s “ID Toolkit” column appeared in the May/June 2015 issue of BirdWatching.