Why a bird must spread a lowered wing, and unequal wing-beats in turning
A dropped wing must open at once or the bird capsizes; turning uses stronger beats on the outer wing
Two short notes on the mechanics of a turning bird. The first observes that when a bird lowers one wing it is forced to spread it again immediately, or it would turn upside down. The second states that in turning the bird does not beat both wings equally, but drives harder the wing tracing the convex (outer) arc than the one tracing the concave arc. Small bird sketches accompany the notes at the upper right.
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A lowered wing must be reopened at once
When the bird lowers one of its wings, necessity compels it immediately to spread that wing out again. If it failed to do so, the bird would roll over and turn upside down. The rule links wing position directly to the bird's stability in the air.
Unequal wing-beats produce the turn
To turn, the bird does not beat its wings with equal motion. It moves more strongly the wing that traces the convex (outer) circle than the one tracing the concave (inner) circle. The difference in effort between the two wings is what curves the flight path.