Bird wings fold primarily by flexing the elbow and adducting the wrist during the upstroke, also known as the recovery stroke, of flapping flight. This precise maneuver is fundamental for efficient aerial locomotion.
The Mechanism of Avian Wing Folding
During active flapping flight, birds exhibit remarkable control over their wing movements to optimize lift and minimize drag. The folding action is a crucial part of this efficiency:
- Timing of the Fold: The primary folding process occurs as the bird moves its wings upwards and forwards, recovering from the powerful downstroke. This phase is scientifically referred to as the upstroke or recovery stroke (Brown 1948, 1953, 1963).
- Key Joint Actions: The specific anatomical movements that enable this intricate fold involve two critical joints:
- Flexing the Elbow: The bird bends its elbow joint, drawing the outer portion of the wing (the forearm and hand) closer to its body.
- Adducting the Wrist: Simultaneously, the wrist joint moves the hand and the large primary flight feathers inward, bringing them closer to the forearm.
This coordinated action effectively shortens and streamlines the wing, significantly reducing air resistance during the recovery phase (Robertson and Biewener 2012).
Why Wings Fold During Flight
The strategic folding of wings during the upstroke serves several vital purposes for a flying bird:
- Minimizing Drag: By reducing the wing's effective surface area and making it more compact, birds drastically decrease aerodynamic drag during the upward and forward motion, which would otherwise hinder forward momentum.
- Efficient Recovery: This folding allows the wing to be repositioned swiftly and effectively for the subsequent powerful downstroke, maximizing the overall efficiency of flight.
- Enhanced Maneuverability: Precise control over wing folding contributes to a bird's agility, enabling it to execute sharp turns and rapid adjustments in its flight path.
This synchronized interplay of joint movements ensures that birds maintain aerodynamic efficiency throughout their complex and diverse flight patterns.
