Birds have the amazing ability to fold their wings against their bodies when not in use. This allows them to reduce drag while flying and also makes it easier to navigate through dense vegetation. Folding wings is an intricate process that requires the coordination of multiple joints and muscles. In this article, we will explore the anatomy behind avian wing folding and look at how different types of birds accomplish it.
Bird wing anatomy
To understand how birds fold their wings, we first need to understand the basic anatomy of the avian wing:
- Humerus – This is the long upper bone of the wing. It connects to the shoulder joint.
- Radius and ulna – These are the two long forearm bones. They rotate to allow folding of the wing.
- Carpometacarpus – This is the palm area of the wing made up of several fused bones.
- Digits – Birds have between three and four fingers or digits which support the primary flight feathers.
- Alula – This is a small digit that projects from the front of the wing and helps reduce turbulence.
- Primary feathers – These large asymmetrical feathers attach to the manus and digits and produce most of the thrust.
- Secondary feathers – Shorter symmetrical feathers that cover the dorsal surface of the wing.
- Coverts – Smaller feathers that cover the bases of the larger feathers.
The humerus, radius, ulna, and carpometacarpus all work together to allow the wing to fold. The digits can also rotate to shift feathers around.
Muscles involved in wing folding
Several muscle groups control the movements required for avian wing folding:
- Pectoralis – This is the large breast muscle that pulls the wing downwards.
- Supracoracoideus – Raises the humerus.
- Coracobrachialis – Pulls the wing forward.
- Extensor and flexor muscles – Extend and flex the digits.
- Abductor and adductor muscles – Abduct and adduct the ulna and radius.
These muscles work in coordination to lift the wing up, rotate it forward, and then pull it in tightly against the body. The digits also contract to point the flight feathers backwards.
Folding in soaring birds
Soaring birds like eagles, hawks, and vultures need to be able to quickly fold their large wings to facilitate perching or making quick turns in the air. Here is how their wing folding process works:
- The pectoralis first pulls the wing downwards from a lifted position.
- The supracoracoideus then raises the humerus parallel to the body.
- The abductors externally rotate the radius and ulna forward so they are perpendicular to the body.
- Finally, the flexors contract to fold the carpometacarpus and digits in tightly against the body.
This folding pattern allows large raptors to quickly ditch drag and maneuver through cluttered environments. The pointed flight feathers also overlap to reduce turbulence.
Folding in other birds
In passerines (perching birds) and waterfowl, the wings usually fold in a more swept back pattern:
- The humerus is pulled back and down against the body.
- The radius and ulna are pronated so the concave surface faces backwards.
- The carpometacarpus is then folded backwards over itself.
- The digits rotate to point the flight feathers against the back and tail.
This method of folding places the feathers in the most streamlined position for reduced drag. In waterfowl, it also allows the wings to be tucked in tightly while swimming.
Alular fold in songbirds
Many passerine songbirds have a specialized alular digit with feathers that allow for a unique folding technique:
- As the wing folds back, the alula remains extended due to a specialized joint.
- The alula feathers then slide over the other primary feathers, covering them.
- This creates a small slotted gap between the folded primaries and the body.
The alular fold likely helps reduce turbulence and drag on the folded wing. Research shows alular folds can reduce drag by up to 15% in some small songbirds.
Wing folding in flight
Birds can actually fold their wings in mid-flight to perform elaborate aerial maneuvers:
- The pectoralis pulls the wing down and supracoracoideus lifts the humerus.
- With the wing partially folded, the bird pitches its body upwards.
- This causes the lift force to swing to the side, turning the bird.
- The wings are then quickly unfolded to stop the turn.
By combining asymmetric wing folding with body pitching, birds can execute tight 180 degree turns within just a few wingbeats. Hummingbirds are especially adept at these complex folding turns.
Takeoff with folded wings
Some birds like swifts and falcons will fold their wings completely against their bodies when diving down to gain speed:
- At the start of the dive, the wings are already folded in tightly.
- This minimizes drag and allows them to accelerate to high speeds.
- Near the bottom of the dive, the wings are quickly extended to generate lift.
- This allows them to pull up and initiate flight with the added airspeed.
Folding in the wings during these stooping dives gives certain birds an extra burst of speed and lift for rapid takeoffs.
Conclusion
The ability to precisely fold their wings gives birds exceptional flight control and efficiency. Each species has adapted the basic wing folding process to suit its specific needs and maneuvers. From hummingbirds to vultures, folding in the wings allows these masters of the sky to exploit every advantage possible in an aerodynamic world. Next time you see a bird landing gracefully on a wire or diving at high speed, take a moment to appreciate the intricate biomechanics behind its folded wings.