Birds’ wings are arguably their most beautiful features. The grace of their form and movement and the amazing range of plumage and color patterns of the avian wing have captivated us throughout history.
More than just fascinating feathered structures, avian wings have evolved to be mechanical masterpieces with complex musculature, joints, and bones. Understanding bird wing anatomy helps us decode the secrets of avian flight and even inspires some of our own aviation technology.
In this in-depth guide, we’ll delve deeper into the structure and function of bird wing anatomy. Read along to learn how birds’ wings enable flight.
A bird's wing is essentially a modified arm or foreleg, complete with familiar structures like the hand, wrist, and forearm. However, adaptations for flight have changed their bones in some remarkable ways.
Starting at the shoulder, we find the humerus, the stout, single bone of the upper wing. The humerus is attached to the major flight muscles, which power the downstroke and recovery (upstroke). After the elbow, the ‘forearm’ is made up of the radius and ulna, which connect to paired wrist bones called the radiale and ulnare. The secondary feathers, which are vital for flight, attach to this section of the wing.
The bird ‘hand’ includes the large, fused carpometacarpus and three digits. The bird’s ‘thumb’ is called the alula and consists of two phalanges (finger bones). The second digit also consists of two phalanges, and the third digit has just a single bone. The mobile primary feathers are located in this part of the wing.
The major muscles that power avian flight are located in the bird’s breast. The largest is the pectoralis major, which pulls the humerus down. This muscle acts antagonistically with the smaller supracoracoideus, which lifts the humerus back up. Birds also bend their wings at the elbow, wrist, and thumb, and these movements rely on the flexing and relaxing of muscles within the wing.
A bird’s wing is completely covered in feathers. Flight feathers (remiges) extend from the tip to the ‘armpit,’ running along the posterior or trailing edge of the wing. The 9-11 large flight feathers on the end of the wing are known as primaries.
A row of smaller flight feathers known as secondaries occurs along the trailing edge of the ulna to the elbow. The rest of the wing is covered in covert feathers (tectrices), and scapulars cover the shoulder area.
Diagram showing the flight muscles of birds
Flapping flight requires a complex combination of extension, flexion, and rotation of several joints rather than just an up-and-down motion. These joints are relatively static while gliding and soaring, and some birds have locking mechanisms to prevent fatigue during long flights.
Continue reading to learn the basics of bird wing articulations.
The humerus articulates with the pectoral girdle at the shoulder. This joint flexes when the pectoralis major contracts and extends with the contraction of the supracoracoideus.
The bird’s elbow is a hinged joint that is held in the flexed position at rest. During flight, birds partially flex this joint to reduce air resistance on the upstroke. The elbow is relatively immobile during soaring flight.
The bird’s wrist joint flexes, bringing the carpometacarpus and other hand bones toward the ulna of the forearm. Ligaments along the forearm help the wrist extend automatically when the elbow is extended.
Flight requires precise coordination of a range of muscles in the bird’s upper body and within each wing. Let’s explore some of these muscles and their role during each wingbeat.
The pectoralis major is responsible for the flexion of the shoulder to power the downstroke, while the supracoracoideus powers the extension of the shoulder to lift the wing. The elbow is flexed by the contraction of the bicep and extended by the contraction of the tricep muscles.
A Gannet in-flight. Flight requires precise coordination of a range of muscles in the bird’s upper body and within each wing
Avian flight requires complex musculature, aerodynamics, timing, and behaviors. However, it would not be possible without fuel and energy transfer from the bird to the air. Let’s follow the basic flow of energy required for flight.
Fatty acids are the bird’s primary fuel source for flight. This energy source obtained from their food, combined with a steady supply of oxygen from an efficient respiratory system, powers the flight muscles.
Muscles in the breast and wings move bones and articulate joints to create the complex motions of flapping flight. Specifically arranged primary and secondary feathers attached to the ulna and hand bones create a streamlined profile but a larger surface area relative to the ground to increase resistance when flapping.
The interlocking barbs and barbules of the flight feathers provide a rigid, airtight surface that imparts a force on the air below them during each downstroke. The result is an equal but opposite reaction that lifts the bird and propels it forward. The airfoil profile of the bird's wing also creates lift as air passes along the upper and lower surfaces of the wing at different speeds.
A Mute Swan. Avian flight requires complex musculature, aerodynamics, timing, and behaviors
Wing loading refers to the ratio of a bird’s body mass to the surface area of its wings. Smaller wings produce a lot less resistance on the downstroke, but they create far less lift, so birds with relatively small wings (high wing loading) must have great stamina to stay aloft. Large wings are more difficult to flap and far less maneuverable, but they allow birds to fly with less effort.
Common Murres have the highest known wing loading of any (flying) bird and must flap their wings rapidly and constantly during flight. The lowest wing loading is found in another seabird with a very different flight style. The Magnificent Frigatebird soars and glides incredible distances with its massive wings.
Wings come in a diverse range of shapes to suit the habitats, foraging behaviors, and migratory patterns of different bird species. They may be long and narrow like the wings of a Gull or short and broad like those of the Broad-winged Hawk.
Avian wing shapes are often placed in the following categories:
Elliptical wings are short and roughly oval in shape. These wings allow fast acceleration and a high degree of maneuverability. They are typical of songbirds and groundbirds.
High-speed wings are found on speedy birds like Swifts, Falcons, and Ducks. They are generally narrow and pointed, creating a highly streamlined, high-aspect-ratio surface to cut through the air.
Soaring and gliding birds harness the free energy of natural air currents to provide the forward (thrust) and upward (lift) momentum required for flight. These birds can travel immense distances and stay airborne while barely flapping their wings.
Gliding seabirds have very high aspect ratio wings that end in points. In contrast, soaring birds like Hawks and Eagles have high-lift slotted wings with a very low aspect ratio. These wings end in a broad, rounded tip with prominent primary feathers.
Birds’ wings are not only useful for flight. They’re also a canvas that some birds use for visual displays and species recognition. Ducks, for example, have colorful reflective patches of plumage on their wings called specula, while many other birds have varied colors on the upper and lower sides of their wings.
Bright colors help birds be seen, but many species prefer to go unnoticed. Their wings may be wonderfully patterned with cryptic neutral tones, and the wings of Nightjars, Woodcocks, and Owls of the Bubo genus are exceptionally well camouflaged.
A Nightjar. The wings of Nightjars, Woodcocks, and Owls of the Bubo genus are exceptionally well camouflaged
Moving through the air requires enough lift to overcome the force of gravity and enough thrust to overcome the resistance of drag. Read on to learn how birds generate these locomotive forces.
While flapping, birds generate thrust with their long primary feathers and lift with both their primaries and secondaries. Birds that fly by constant flapping may have elliptical, high-speed, or high-lift wings.
The large, broad wings of heavy-bodied birds like Swans and Bustards are flapped relatively slowly, resulting in a slow and steady progression. Birds with higher wing loading are sprinters that flap their wings at high speeds.
Gliding and soaring birds have evolved to cheat the force of gravity by harnessing the energy in natural air currents. These birds rely on their light frame, large wing surface area, and the natural lift provided by their airfoil-shaped wings.
Relatively few birds have mastered the art of hovering, which requires balancing thrust with drag and lift with weight. Hummingbirds achieve this feat with rapid wingbeats in a figure-of-8 pattern.
A bird’s flight feathers are perfectly evolved to provide a large surface area in a lightweight package. They have both rigid and flexible properties that come in handy during the various phases of the wing stroke.
Birds’ feathers pivot as their wings extend and flex, changing the amount of air resistance and allowing them to control the direction of thrust and maximize lift generation. During the downstroke, the flight feathers adhere together to prevent air from passing between them and maximize lift efficiency. On the upstroke, the primary feathers twist and separate, allowing air to pass between them.
As a bird flies, air below the wing generates a high pressure, which passes over the wing tips to the low-pressure air above. The resulting vortices and turbulence of the rushing air create a slowing effect called induced drag.
Fast-flying and gliding birds have evolved narrow, pointed wings to minimize this form of drag. However, many birds reduce aerodynamic drag with a completely different strategy.
Broad-winged birds like Harris’ Hawks have prominent primary feathers that project like fingers from the wing tips. These feathers can be separated vertically to form slots that reduce turbulence and increase energy efficiency.
Broad-winged birds like Harris’ Hawks have prominent primary feathers that project like fingers from the wing tips
From intricate and delicate feather structures to powerful flight muscles, each anatomical component of the avian wing is vital for sustained flight.
Birds may differ wildly in size, shape, and appearance, but almost all the surviving species have evolved wing anatomy and flight techniques to suit their habitat and lifestyle.
Even with such advanced wing anatomy, birds can’t escape the increasing threats of habitat destruction, pollution, and persecution. Look a little longer next time you spot a bird in flight - we should all appreciate the value our feathered friends bring to our lives and the global ecosystem.
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