Flapping flight is the most common form of bird flight. It involves birds generating both lift and thrust by flapping their wings. The flapping of wings provides the necessary aerodynamic forces for birds to stay aloft, maneuver, and propel themselves forward through the air.
What causes birds to flap their wings?
Birds flap their wings to produce lift and thrust. Lift counteracts the bird’s weight and keeps the bird aloft. Thrust provides a force to overcome drag and propel the bird forward. Without flapping, a bird would quickly stall and fall from the sky. Flapping the wings generates aerodynamic forces in the following ways:
- The downward motion of the wings deflects air downward, resulting in a reaction force upward known as lift.
- The angle of the wings causes air to flow faster over the top surface, decreasing pressure and helping lift the wings.
- The backward motion of the wings during the upstroke pushes air backward, providing thrust.
- Varying the speed and angle of attack alters the balance between lift and thrust, allowing maneuverability.
Flapping flight allows birds to produce both lift and thrust across a wide range of speeds and perform aerobatic maneuvers impossible with gliding flight.
What muscles do birds use to flap their wings?
Birds have powerful chest muscles called the pectoralis major and supracoracoideus that are responsible for downstroke wing flapping. Birds also utilize secondary flight muscles in the chest, back, and wings for upstroke flapping and fine wing control.
The most important flight muscles are:
- Pectoralis major – the large fan-shaped muscle of the chest, responsible for around 70-80% of flapping power.
- Supracoracoideus – a smaller chest muscle that assists the pectoralis major.
- Subscapularis – a back muscle that raises the wing during upstroke.
- Wing flexors and extensors – manipulate the wing shape during flapping.
These muscles have a high proportion of oxidative fast-twitch fibers for stamina and the rapid contraction needed for flapping wings up to 80 times per second.
How does wing shape affect flapping?
Bird wing shapes are adapted for different types of flapping flight. Key features that influence flapping ability include:
- Wing area – Larger wings produce more lift and thrust.
- Wing length – Long, narrow wings are efficient for long-distance soaring and gliding.
- Wing loading – The ratio of body weight to wing area. Higher wing loading requires faster flapping and is typical of agile predatory birds.
- Aspect ratio – The ratio of wing length to breadth. High aspect ratio wings generate more lift and are adapted for migratory flight.
- Wing shape – Pointed, crescent-shaped wings provide fast acceleration and tight maneuvering.
Wing shape adaptations allow different birds to perform specialist roles – from hummingbirds that can hover and fly backwards, to albatrosses designed for dynamic soaring over oceans.
Phases of the Flapping Cycle
There are two major phases to the flapping cycle that allow birds to produce lift and thrust:
Downstroke
The downstroke is the active power phase of flapping:
- Birds use their powerful chest muscles to depress the wing downwards.
- Air is pushed downward, generating lift.
- Airflow over the wing provides additional lift force.
- The backward sweep of the wing pushes air rearwards, producing thrust.
Over 70% of the lift and thrust are produced during the downstroke. The wing sweeps downwards in a sculling motion at an angle of attack of around 10-15 degrees to the body. To maximize power, the wings are extended fully and often twisted or turned through the stroke.
Upstroke
On the upstroke, birds usually flex their wings to reduce resistance as the wing is brought forward and upward. Lift and thrust production are minimized, though some birds make an active upward sweep to provide additional lift, at the cost of higher energy expenditure.
Key features of the upstroke phase:
- Wings are held at a positive angle of attack but generate less than 30% of total lift.
- Wings may be flexed or swept forward to minimize drag.
- The upstroke produces very little thrust, and may even incur some backward drag.
- Birds can perform a powered upstroke by spreading the wings to produce additional lift and thrust.
Varying the upstroke allows birds to control speed and maneuverability. A flexed upstroke favors speed while an extended upstroke allows tight maneuvering at low speeds but requires more energy.
Transition Phases
At the end of each stroke, the wings rotate and transition to the next phase:
- Supination – the wings twist to raise the leading edge for upstroke.
- Pronation – the wings rotate to lower the leading edge in preparation for downstroke.
These transition phases are very short, accounting for less than 10% of the total stroke cycle, but allow the continuous changeover between upstroke and downstroke.
Flapping Flight Speed and Stroke Patterns
Birds can manipulate their flapping cadence and style to fly at different speeds. Some typical flapping profiles include:
Flight Speed | Wing-beat Frequency | Stroke Pattern |
---|---|---|
Hovering | Around 80 beats/second | Relatively symmetric up and downstrokes generating weight support and thrust. Requires high power. |
Slow flight | 10 – 30 beats/second | Deep, powerful downstroke with flexed passive upstroke. Allows slow maneuverable flight such as takeoff and landing. |
Cruising flight | 30-50 beats/second | Moderate upstroke and downstroke balance. Provides efficient long distance flight. |
High speed flight | Over 50 beats/second | Shallow, stiff-winged flapping minimizing drag. Upstroke may be eliminated. |
Wing-beat Frequency and Amplitude
Wing-beat frequency, or number of flaps per second, tends to increase linearly with flight speed until reaching an upper power limit. Smaller birds flap their wings faster than larger birds.
Wing-beat amplitude – the vertical excursion of the wingtips – tends to decrease at higher flight speeds. Birds flap with greater amplitude at slow speeds and smaller amplitude when cruising or maneuvering.
By tuning wing-beat frequency and amplitude, birds can fine-tune the lift produced for different flight requirements.
Mechanics and Aerodynamics of Flapping
The flapping wings of birds exploit various aerodynamic mechanisms to generate high lift and thrust forces. These include:
Leading Edge Vortices
At moderate to high angles of attack, a spiral vortex of air forms over the wing’s leading edge, creating low pressure and additional lift. This vortex increases lift substantially – especially during slow flights.
Wake Capture
During the downstroke, the wings trap horizontal air vortices generated during previous strokes. This trapped air gets accelerated downward, amplifying downward thrust production.
Rotational Circulation
As the wings pronate and supinate, rotational forces increase the speed of air flowing over the wing, enhancing lift generation.
Clap and Fling
Some birds clap their wings together during upstroke or fling them apart at the top of upstroke to smooth airflow and boost lift.
Weaving
Horizontal weaving motions during flapping may produce aerodynamic forces for maneuvering and control.
Gust Response
Birds actively adjust wing flexion and angle of attack to respond to air gusts, maintaining stability. This is mediated by reflexes in the wing muscles.
Kinematics and Morphing Shape of Flapping
Birds dynamically morph their wings during flapping in complex ways to generate and control aerodynamic forces. Important kinematic mechanisms include:
Wing Flexion
During upstroke, the hand wing is often flexed upward at the wrist to reduce drag. During downstroke, the wing locks flatten and stiffen for power generation.
Wing Twisting
The wing angle of attack twists along the length of the wing dynamically through each stroke. This controls airflow and vorticity patterns.
Feather Spreading
Birds actively spread their primary feathers apart to increase wing area on downstroke, and then pull them together tightly on upstroke.
Spanwise Bending
The leading edge may bend or flex upwards spanwise along the wing to control airflow and delay stall at high angles of attack.
Alula Fluttering
The alula feathers at the wing leading edge pop up to increase lift, then flatten to decrease drag, around 100 times per second.
By morphing wing shape in complex ways harmonic with the flapping cycle, birds achieve exquisite control over their aerodynamics and flight forces.
Evolution of Avian Flight
Birds evolved from feathered dinosaurs over 150 million years ago. Flapping flight likely evolved incrementally from precursors in non-avian dinosaurs:
Stage 1 – Feathered forelimbs
Proto-wings evolved for insulation, display, or prey capture before flight ability.
Stage 2 – Gliding and parachuting
With enlarged feathered forelimbs, dinosaurs may have employed parachuting from trees or cliffs. This selects for aerodynamic improvements.
Stage 3 – Controlled flapping descent
Powered downstroke flapping could have preceded sustained flight, allowing controlled descents and stability similar to modern juvenile galliform birds.
Stage 4 – Burst flight ability
With further refinements to wings and muscles, protobirds evolved brief upward flight after leaping from the ground or trees.
Stage 5 – Sustained flapping flight
Finally, true flapping flight evolved in early birds like Archaeopteryx, with aerodynamic wings, asymmetric flight feathers, and a keeled sternum for large flight muscle attachment.
Modern birds retain anatomical traces of their evolutionary origins, like flightless ratite birds with reduced keeled sternums. The incremental evolution of flapping flight opened up new ecological opportunities for the ancestors of modern birds.
Constraints and Trade-Offs in Flapping Flight
While flapping flight provides birds great flexibility, there are certain trade-offs and constraints:
Power Requirements
Flapping flight requires huge power outputs. Birds have very high metabolic rates compared to similar sized mammals, and must consume large quantities of oxygen and energy to sustain flapping. This constrains duration, range, and payload capacity.
Scaling Limits
As birds increase in size, flapping flight becomes less feasible. The power required to flap wings increases faster than the power output from flight muscles as size increases. This limits maximum useful size.
Maneuverability Versus Stability
Long, high aspect ratio wings provide greater efficiency and range, but short, broad wings allow superior maneuverability. Birds must strike a balance appropriate for their ecology.
Flapping Versus Soaring
Many large birds rely more on gliding and soaring over flapping flight to conserve energy. But this reduces maneuverability and requires specific soaring conditions.
Muscle Fraction
Birds must devote a large fraction of their body mass to flight muscles, limiting space for organs and cargo. Flightless birds are freed from this constraint and can devote more mass to egg-laying, for example.
Evolution has shaped an optimum middle ground in each species balancing the costs and benefits of sustained flapping flight.
Concluding Thoughts
In summary, flapping is an elegant and complex biomechanical system that gives birds access to the third dimension. By flapping their wings, birds can not only fly, but also precisely control speed, direction, and even upside down or backwards maneuvers. Understanding the mechanics, aerodynamics, and evolution of flapping is key to appreciating avian flight in all its diversity. From hummingbirds to the extinct giant Argentavis with a 7 meter wingspan, flapping flight allows birds to inhabit aerial niches unavailable to other organisms.