When birds fly, their wings flap in an up and down motion. This allows them to generate lift and thrust to stay aloft and propel themselves forward. The flapping motion produces aerodynamic forces that counteract the force of gravity and enable the bird to fly.
Why do birds flap their wings up and down?
Birds flap their wings up and down because this motion allows them to produce the aerodynamic forces needed for flight. As the wing moves downward, air flows faster over the top of the wing, which decreases air pressure above the wing. At the same time, air flows slower under the wing, increasing air pressure below. This difference in pressure results in an upward force called lift. As the wing flaps up, it pushes air downward, generating thrust to propel the bird forward.
Flapping up and down allows birds to continuously generate lift and thrust. If the wings simply glided through the air, they would not produce enough aerodynamic force to stay airborne. The flapping motion keeps air moving over the wings to maintain the pressure difference and associated upward lift force. The repetitive flapping also provides constant thrust to overcome drag and propel the bird along.
Wing shape and angle of attack
In addition to flapping up and down, birds can alter the shape and angle of their wings to optimize lift and thrust production. As the wing flaps down, birds often angle it slightly upwards, increasing the angle of attack. This gives the air flowing over the top of the wing a longer path, increasing the speed of airflow and decreasing pressure to produce more lift. On the upstroke, birds rotate the wing to decrease the angle of attack, reducing resistance and allowing them to flap up with less effort.
The wings of birds are also shaped to enhance lift generation. The top surface of the wing is curved while the underside is relatively flat. This shape deflects air downward as it flows over the curved top surface. According to Bernoulli’s principle, when air travels faster over the curved top it results in lower pressure, generating more upward lift force.
Hovering, gliding and maneuvering
By modulating the speed, angle and force of their wing flapping, birds can produce enough lift and thrust to hover, glide and maneuver in diverse ways. Very fast flapping enables hummingbirds to generate weight support and precise control for sustained hovering. Large birds like hawks and eagles can rely on occasional flapping combined with wings held at an angle to ride air currents and glide with little effort. Maneuverable fliers like swallows can alter the symmetry of left and right wing strokes to execute tight turns.
The up and down flapping motion of birds provides the key to their remarkable flight capabilities. Producing aerodynamic forces essential for overcoming gravity and propulsion, wing flapping allows birds to fly in a vast array of aerial styles.
What are the different phases in a flapping cycle?
The flapping cycle of a bird’s wings consists of two major phases: the downstroke and upstroke. Within each wing stroke, there are finer movements birds utilize to optimize lift and thrust.
Downstroke
As the wings flap down, they go through the following movements:
- Pronation – the wing rotates forward so the leading edge points downward. This helps angle the wing to generate more lift.
- Depression – the wing moves downward, pushing air and producing thrust.
- Flexion – the wrist joint flexes to change wing curvature and angle of attack.
Upstroke
As the wings flap up, the following motions occur:
- Supination – the wing tilts back so the leading edge points upward.
- Elevation – the wing moves upward through the air.
- Extension – the wrist joint extends to unflex the wing.
Additionally, many birds fold their wings in toward their bodies during the upstroke. This reduces resistance and allows them to flap up with less exertion.
Transition phases
There are also transition phases that smooth the switch between downstroke and upstroke:
- Pronation – at the top of the upstroke, the wing rotates forward to transition into the downstroke.
- Supination – at the bottom of the downstroke, the wing rotates back to begin moving upward.
By coordinating motions throughout the entire flapping cycle, birds generate aerodynamic forces in an energy efficient manner. The details of wing motion are precisely tuned to enable different flight capabilities from hovering to gliding.
How many times per second do different birds flap their wings?
The flapping frequency of birds varies widely depending on their size and flying style:
| Bird Type | Flaps per second |
|---|---|
| Hummingbird | 20 to 80 |
| Songbirds | 5 to 15 |
| Seagulls | 3 to 5 |
| Vultures | 1 to 3 |
In general, smaller birds flap their wings much faster than larger birds. For example, hummingbirds can flap up to 80 times per second, while large vultures flap lethargically at around 1 to 3 times per second. This is because the aerodynamics of flapping flight require less effort for smaller wings that produce lower forces. Larger, heavier birds cannot accelerate their wings as rapidly and must rely more on gliding.
Birds that specialize in high speed, agile flight like swifts and swallows also tend to have higher flapping frequencies, often around 10 times per second. Slower flying birds like gulls and herons flap their wings at 3 to 5 times per second during cruising flight. But all birds adjust their flapping frequency as needed for maneuvers or gaining altitude.
The tiny wings of hummingbirds allow them to flap extraordinarily fast. Yet their wing aerodynamics generate the forces necessary to hover and accelerate rapidly. At the other extreme, large soaring birds hardly need to flap their imposing wings to utilize air currents and span great distances on their migrations.
How does wing size and shape affect flapping?
The size and shape of a bird’s wings have significant impacts on its flapping behavior and flight capabilities. Birds with different wing characteristics flap their wings in various ways adapted for their ecology and behavior.
Wing loading
Wing loading refers to the ratio of a bird’s weight compared to its wing area. Birds with higher wing loading need to flap faster to generate enough lift for flight. For example, chickens have a high wing loading and must flap almost constantly to stay airborne. Birds of prey like eagles have lower wing loading and can glide more easily between infrequent flapping.
Wing aspect ratio
The aspect ratio of a wing compares its length to its chord (width). Long, narrow wings with high aspect ratios provide performance benefits for flapping flight. Birds like albatrosses have long, slender wings for energy efficient soaring and gliding across oceans. Short, broad wings produce more drag but allow for greater maneuverability and hovering.
Wing shape
Pointed, slotted, crescent and elliptical are some of the wing shapes adapted by different birds. Raptors often have broad wings with slotted tips to reduce drag and enable graceful gliding. Hummingbirds have pointed wings specialized for hovering and rapid maneuvering. Seabirds may have narrow, crescent-shaped wings to excel at gliding dynamically over wave fronts.
Wing twisting
Birds can dynamically twist their wings during flapping to optimize airflow. For example, twisting the wing tips upward on the downstroke can reduce drag. Many large birds rely on gulping flight in which the wings are furled, twisted and unfurled during each stroke to assist climbing flight.
Modulating flapping
Birds constantly modulate the speed, force and direction of their flapping throughout a wingbeat. The complex motions involved in pronating, supinating, flexing and extending allow birds to turn, maneuver and manipulate their flight dynamics.
By adapting their wings through evolutionary timescales or modulating wing motions in real time, birds precisely tune flapping flight for their lifestyles and navigation of diverse habitats.
How does wind affect a bird’s flapping?
Wind conditions have significant effects on a bird’s flapping flight. Birds alter their flapping frequency, force and wing angles to compensate for headwinds, tailwinds and gusts.
Headwinds
Flying into a headwind, birds increase their flapping frequency and power to maintain forward momentum. The wings flap faster to generate extra thrust that overcomes the countervailing wind. Birds may also raise their body angle to direct more lift force forward against the wind.
Tailwinds
With a tailwind behind them, birds can reduce flapping frequency and glide more. The wind provides free thrust, allowing the wings to flap less frequently. Soaring birds like albatrosses extensively utilize tailwinds to travel rapidly over huge distances with minimal energetic cost.
Gusts and turbulence
When buffeted by gusts or turbulence, birds alter the angle of attack and twist of their wings to maintain stability and control. The wings may flex and extend to smoothly ride out problematic airflows. Flapping frequency and force can modulate to counteract gusts. Birds may also change their body orientation relative to the wind to stay on course.
Skillful flapping responses enable birds to exploit helpful winds and deftly negotiate challenging conditions. Analyzing how birds flap and glide in diverse winds provides models for engineers designing autonomous flying robots.
How does flapping enable different flight modes?
By modulating their flapping mechanics, birds can achieve various flight modes including hovering, gliding, soaring, and maneuvering.
Hovering
Hummingbirds and kingfishers can hover in place by flapping their wings rapidly back and forth. At around 10-15 meters per second, the wing speed generates enough lift to counteract gravity and keep the bird suspended mid-air with no forward motion.
Gliding
After a powered ascent, birds can enter a glide by holding their wings outstretched and eliminating flapping. This allows them to descend gradually over a horizontal distance with minimal energy expenditure. The wings generate lift to slow the descent while also providing some thrust to maintain speed.
Soaring
Large birds like eagles, hawks, vultures and albatrosses enter soaring flight upon finding rising columns of warm air called thermals. They cease flapping and ride the uprising air which keeps them aloft, expending very little energy. This enables extremely long distance travel powered by ambient winds and thermal updrafts.
Takeoff and landing
During takeoff, birds utilize rapid, powerful flapping to launch themselves into the air. Once airborne, flapping frequency decreases for cruising flight. Landing requires modulating the wings to control descent and braking. Flapping can essentially reverse thrust near the ground for a gentle touchdown.
Maneuvering
Sharp turns, dives, rolls and other aerial maneuvers require asymmetric flapping. For example, during a banked turn, birds angle the wings differently on each sidestroke. One wing flaps at a steeper angle to provide extra lift while the other generates less lift. Careful modulation of the wings enables extreme precision and agility.
From hovering hummingbirds to migrating albatrosses, flapping flight equips birds with extraordinary behavioral diversity. By flexibly adjusting wing motion, birds achieve feats of flight unmatched by human engineering.
How did flapping evolve in birds?
Flapping flight evolved in small theropod dinosaurs through a series of adaptations over tens of millions of years. Fossil discoveries have revealed how these proto-birds developed modern flight biomechanics.
Feathered wings
Feathers likely first evolved for insulation but gradually became adapted for aerodynamic purposes. The feathers along the arms of bipedal theropods eventually formed large surfaces ideal for flapping flight. Fossils like Microraptor depict feathered proto-wings on dinosaur limbs.
Lightweight skeletons
Becoming smaller and lighter was essential for proto-birds to achieve flight. Fossils document the evolution of skeletal pneumaticity – air sacs in the bones – that reduced weight. Streamlined, fused, and hollowed bones aided aerodynamic performance.
Powerful flight muscles
The evolution of large, powerful pectoral muscles drove flapping ability. Flight strokes became stronger and faster by muscular adaptations across the chest, shoulders and back. This enabled generation of the high lift and thrust forces required.
Improved lungs & circulation
Active flapping flight requires extensive cardiovascular adaptations to supply muscles with oxygenated blood. Enhanced lung capacity, efficient air sacs and a four-chambered heart with high metabolism evolved in proto-birds.
Computer-like brains
The complex neurological and proprioceptive feedback needed for controlled flapping flight spurred avian brain evolution. Their sophisticated brains coordinate precise wing articulation throughout flapping phases.
From dinosaurlike ancestors, birds gradually evolved excellent flapping biomechanics. Over 150 million years, they perfected the diverse modes of flight we see today, from hummingbird hovering to albatross soaring.
Conclusion
The characteristic up and down flapping motion of bird wings provides the thrust and lift forces necessary for performing a wide array of aerial behaviors. Differences in flapping frequency, wing shape, angle of attack modulation and other biomechanical factors allow diverse birds to occupy niches from hummingbird-pollinated flowers to transoceanic migrations. Continued study of the nuances of flapping flight across species provides inspiration for engineers seeking to mimic bird maneuverability in robotic aircraft. As engineers delve deeper into principles refined by evolution, they edge closer to achieving true bird-like flight capabilities.
