Flapping flight is the most common form of avian locomotion and is used by nearly all species of birds. It involves the flapping of the wings to generate both lift and thrust, allowing birds to fly through the air. Unlike gliding flight, where birds rely on air currents to stay aloft, flapping flight allows self-powered and controlled flight. There are many complex aerodynamic and biomechanical factors involved in sustaining flapping flight.
What are the key features of flapping flight?
There are several key features that enable birds to achieve flapping flight:
Wings
Birds have specially adapted forelimbs called wings that provide the lift and thrust required for flapping flight. Wings have evolved from the basic theropod dinosaur forelimb but are optimized for powered flight. They contain flight feathers called remiges that form the airfoils that provide lift. The asymmetrical shape of the wings produces aerodynamic forces when flapped.
Lightweight skeleton
Birds have an extremely lightweight skeleton with hollow, pneumatized bones. This minimizes body weight while retaining bone strength. The streamlined, fusiform shape also decreases drag. These weight savings allow easier flight.
Powerful flight muscles
Birds have massive and powerful pectoral muscles, called the supracoracoideus and pectoralis, which control the downstroke of the wings. They make up 15-25% of a bird’s total muscle mass. These provide the power output needed for flapping flight.
Articulated shoulder joint
The shoulder joint has great mobility that permits wing movements through a wide arc. The glenoid fossa is oriented to facilitate the largely anteroposterior plane flapping movements.
Asymmetrical wing feathers
The aerodynamic shape of the flight feathers generates lift on the downstroke but reduces drag on the upstroke. This asymmetrical configuration maximizes the net aerodynamic forces during the whole wingbeat cycle.
Reduced tail
Most birds have a reduced, shortened tail with fewer tail feathers (rectrices) compared to their theropod ancestors. This reduces weight and drag. The fan of tail feathers helps provide stability and control in flight.
What are the phases of the flapping cycle?
There are two major phases in the flapping cycle that generate lift and thrust:
Downstroke
As the wings are flapped downwards, this produces positive lift and thrust. The pectoralis muscles are contracted to depress the wings. The wings are oriented at a positive angle of attack which deflects air downwards. According to Newton’s third law, the air exerts an equal and opposite upward force on the wings providing lift. Thrust is also generated as the wings push rearwards against the air.
Upstroke
On the upstroke, the wings are raised upwards. This produces negative lift but the bird exerts power to overcome drag. The supracoracoideus lifts the wing. The wing is oriented at a negative angle of attack to reduce aerodynamic resistance. No thrust is produced during this phase. At the end of the upstroke, the wings may be brought together above the bird’s back in a clap to reduce energy loss.
This cyclical pattern of downstroke and upstroke in sequence provides both the lift and thrust forces needed to sustain flapping flight. The wings flap at angles of attack of up to 45 degrees during the downstroke. Flapping frequency depends on the size of the bird but is typically between 5-15 beats per second.
How do birds control flapping flight?
Birds can precisely control flapping flight by modulating the wingbeat pattern and using their flight feathers:
Asymmetrical strokes
By independently adjusting the movement of each wing, birds can perform asymmetrical downstroke and upstrokes. This allows them to execute banking maneuvers and turns in flight.
Wingbeat frequency
Increasing or decreasing the flapping frequency changes the power output. Higher frequencies provide more lift and thrust for takeoff and climbing flight. Reducing frequency can induce stalling and controlled descent.
Wing stroke amplitude
Varying the vertical excursion of the wing strokes changes the aerodynamic forces produced. Greater amplitude generates more lift at lower flight speeds like takeoff.
Angle of attack
Altering the angle between the wing chord and the oncoming air modifies lift. Different angles are used for takeoff, cruise, and landing. Birds can make rapid adjustments.
Wing surface area
Extending or retracting the wing feathers increases or decreases the functional surface area of the wings. This is used to make fine adjustments to the lift.
Tail spreading
Fanning or closing the tail feathers controls drag and stability. Wide tails increase drag and allow more maneuverability.
How do birds take off and land in flapping flight?
Birds use specialized flapping patterns and body positions for launch and landing:
Takeoff
To take off from a surface, birds stand with their wings spread to gain maximum lift. They flap with increasing frequency and amplitude. The wings generate high lift and thrust to accelerate from rest and launch into the air. The legs provide additional thrust as they push off from the ground.
Landing
When landing, birds raise their wings and tail to increase drag and reduce airspeed. The wings are flapped at lower frequencies but increased amplitudes to provide additional drag and maintain lift until just before touchdown. Just before landing, the legs are lowered and the wings pronated to stall for a light landing.
Perching
Many small birds can perform vertical or even upside takeoffs and landings when perching using their wings to both provide lift and grip the perch. High angles of attack and lift are generated as the approach the perch for controlled landings.
How do birds perform complex maneuvers in flapping flight?
The agility and versatility of flapping flight allows birds to perform complex aerial maneuvers:
Turning in flight
Banked turns are executed by asymmetrically adjusting the wing strokes. One wing flaps at higher amplitudes and angles. This banks the lift force and rotates the bird into the turn. The tail may spread to resist adverse yaw.
Hovering
In hover feeding, birds like hummingbirds can flutter their wings at high frequencies to generate enough lift and thrust to remain stationary relative to the ground. The body is upright and tail spread.
Backward flight
During backward flight, birds reverse the direction of the wing strokes so that the thrust is directed forwards to propel themselves backwards. This may protect their eggs.
Takeoff from water
To take off from water, birds like swans run along the surface flapping their wings to build up airspeed. This provides the lift needed to get airborne despite the lack of solid ground beneath their feet.
Landing on water
When landing on water, seaplanes use their feet like pontoons to provide floating support and drag while slowing down with fins fully spread. This allows a gentle touchdown without stalling.
How did flapping flight evolve in birds?
Flapping flight evolved in birds’ theropod dinosaur ancestors over hundreds of millions of years through incremental adaptations:
Feathered forelimbs
Feathered wings on maniraptoran dinosaurs may have initially provided insulation, display, or limited gliding capacity. This provided a basis for subsequent evolution into aerodynamic surfaces.
Co-opted for flight
With progressive evolutionary changes to their anatomy and musculature, protobirds like archaeopteryx eventually gained the ability for flapping flight. Their feathered forelimbs became adapted for lift generation.
Refined skeletal support
Key skeletal changes like the furcula (wishbone), keel on the sternum, and fused hand bones provided rigid anchors for large flight muscles and improved weight distribution for flight.
Powerful flight stroke
The large, asymmetric flight muscles and shoulder mobility developed to enable an effective downstroke that provided thrust and lift in early flapping flight.
Aerodynamic feathers
Asymmetrical flight feathers and alulae evolved later refinements that enhanced control and efficiency in flapping flight for modern birds.
Conclusion
In summary, flapping flight in birds is made possible by specialized adaptations like wings with flight feathers, a lightweight skeleton, and powerful musculature. Key phases in the flapping cycle provide the aerodynamic forces of lift and thrust. By modulating the wingbeat kinematics and feather configurations, birds can precisely control complex aerial maneuvers. Flapping flight in birds evolved incrementally over millions of years from theropod dinosaur ancestors into the diverse and sophisticated fliers we see today. Understanding the mechanics and evolution of flapping flight provides insights into aerodynamics, biomechanics, and the history of life on Earth.
