Logan Steele
Birds have strong muscles that enable them to fly, with some species having wings that make up a third or more of their body weight. The pectoralis muscle, which lowers the wing, and the supracoracoideus muscle, which raises it, are the two pairs of large muscles responsible for producing the mechanical power required for sustained flapping flight. The muscle composition varies between species and even within families, with some birds having stronger muscles for manoeuvring flight behaviours and others with smaller muscles for adjusting wing orientation. Birds also have abdominal muscles that expand and contract the chest and hold the ribcage, as well as unique necks with complex musculature that allows the head to perform functions that other animals may use pectoral limbs for.
| Characteristics | Values |
|---|---|
| Muscle composition | Varies between species and even within families |
| Muscle composition and adaptation | Theories of muscle adaptation differ between flapping and gliding flight |
| Muscle function | The pectoralis and supracoracoideus are responsible for producing the power required for sustained flapping flight |
| Muscle power output | Can be used to assess how whole-body power output and aerodynamic power output vary with flight condition and speed |
| Muscle force | Obtaining accurate measurements is challenging due to limitations in existing methods |
| Muscle size | Wings have large muscles that can account for a third or more of a bird's body weight |
| Muscle structure | The supracoracoideus muscle achieves a pulley-like action by using a tendon that passes through the coracoid, furcula, and scapula |
| Muscle location | The supracoracoideus muscle is located beneath the wing, unlike in conventional anatomy where it is believed a muscle above the wing is needed to raise it |
| Muscle movement | The pectoralis muscle lowers the wing, while the supracoracoideus muscle raises it |
| Muscle types | Avian striated muscles contain a respiratory pigment called myoglobin, which is more prevalent in "dark meat" |
| Muscle respiration | Birds use intercostal and abdominal muscles for respiration, unlike mammals that use a diaphragm |
| Muscle groups | Birds have abdominal muscles that expand and contract the chest, and hold the ribcage |
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What You'll Learn
Birds have strong respiratory muscles
The avian respiratory system differs from that of other land vertebrates in that several pairs of nonvascular air sacs are connected to the lungs and extend into the pneumatic parts of the skeleton. The active phase of respiration in birds is exhalation, which requires the contraction of their respiratory muscles. Relaxation of these muscles causes inhalation. Three distinct sets of organs are involved in respiration: the anterior air sacs, the lungs, and the posterior air sacs. The number of air sacs can vary between seven and twelve, depending on the bird species.
The avian lung also differs from other land vertebrates in that it contains a network of microscopic tubes called "neopulmonic parabronchi." These tubes branch off from the posterior air sacs and open into the dorso- and ventrobronchi, as well as directly into the intrapulmonary bronchi. The air flow in the neopulmonic parabronchi is bidirectional, in contrast to the unidirectional flow in the parabronchi. However, the neopulmonic parabronchi only make up a small portion of the total gas exchange surface in birds, typically less than 25%.
In addition to the respiratory muscles, birds also possess abdominal muscles that expand and contract the chest, as well as a unique neck musculature that allows for a wide range of head movements. The muscle composition and adaptation vary between different bird species and even within families.
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The pectoralis major is a prominent breast muscle
The pectoralis major is a thick, fan-shaped or triangular convergent muscle of the human chest. It is the most prominent and largest muscle of the anterior chest wall, lying underneath the breast tissue. The pectoralis major is also referred to as "pecs" or "chest muscle" in colloquial terms.
The pectoralis major has two heads, the clavicular and the sternocostal, which originate from different areas. The clavicular part is close to the deltoid muscle and contributes to the flexion, horizontal adduction, and inward rotation of the humerus. On the other hand, the sternocostal part is responsible for the downward and forward movement of the arm, as well as inward rotation during adduction. The muscle's primary functions are flexion, adduction, and internal rotation of the humerus.
The pectoralis major is a crucial muscle for shoulder joint movement and plays a role in keeping the arm attached to the body. It arises from parts of the clavicle, sternum, costal cartilages of the true ribs, and the abdominal external oblique muscle. The muscle receives dual innervation from the medial and lateral pectoral nerves.
In rare cases, the pectoralis major muscle may develop intramuscular lipomas, which are benign tumours that resemble enlarged breasts. These tumours can be accurately identified using advanced imaging techniques such as computed tomography and magnetic resonance imaging (MRI). Additionally, Poland Syndrome is a rare congenital condition characterised by the absence of the pectoralis major muscle, often accompanied by the absence of the breast in females.
The pectoralis major is also relevant in the field of medicine, particularly in breast surgery. Procedures such as modified radical mastectomy, breast reconstruction, and augmentation can pose risks to the nervous supply of this muscle. Furthermore, the pectoralis major has been utilised in neck surgery and soft tissue reconstruction due to its size, location, and blood supply.
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Birds have striated muscles
Birds have evolved from reptilian ancestors, but the exact lineage is still debated. Some scientists believe they evolved from thecodont reptiles, while others argue that they descended from theropod dinosaurs. Birds have a light but powerful musculature, with around 175 different muscles, mainly controlling the wings, skin, and legs. The muscle mass is concentrated ventrally, with the largest muscles being the pectorals, which control the wings and make up about 15-25% of a flighted bird's body weight.
The pectoralis muscle is the dominant avian flight muscle, providing the powerful wing stroke essential for flight. It is attached to the head of the humerus and, in most birds, it is much larger than the supracoracoideus muscle, which lies beneath it. However, in some groups that use a powered upstroke of the wings, such as penguins, auks, swifts, and hummingbirds, the supracoracoideus is relatively large. These two muscle groups work together to produce the mechanical power required for sustained flapping flight.
The respiratory system of birds is also adapted to support their high metabolism and flight capabilities. Birds lack a diaphragm, so they use their intercostal and abdominal muscles to expand and contract their thoraco-abdominal cavities, changing the volumes of their air sacs. This results in exhalation, which is the active phase of respiration in birds, requiring contraction of their muscles of respiration. The relaxation of these muscles causes inhalation.
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Bird muscle composition varies between species
The muscle composition of birds varies between species, and even within families. Birds have a light skeletal system and powerful musculature, which, along with their circulatory and respiratory systems, enable them to fly. The number of hollow bones varies among species, although large gliding and soaring birds tend to have the most. Diving birds, such as penguins, loons, and puffins, have less hollow bones than non-diving species.
The cardiac and smooth muscles of birds are similar to those of reptiles and mammals. The smooth muscles in the skin include minute feather muscles, which help with flight maneuvers and mating rituals. The striated muscles that move the limbs are focused on the girdles and proximal parts of the limbs. The pectoralis muscle, which is the dominant avian flight muscle, and the supracoracoideus muscle are responsible for producing the mechanical power required for sustained flapping flight. The pectoralis muscle lowers the wing, while the supracoracoideus muscle raises it. The supracoracoideus muscle is attached to the keel of the sternum and connects to the top of the humerus by a pulley, a mechanism unique to vertebrates.
The avian lung and air sac system differ from those of other land vertebrates. Several pairs of nonvascular air sacs are connected to the lungs and extend into the pneumatic parts of the skeleton. The number of air sacs can range from seven to twelve, depending on the species of bird. The avian digestive system has also adapted to accommodate a high metabolic rate and flight. The stomach is typically divided into the proventriculus and the gizzard, the latter of which lies near the center of gravity of the bird and compensates for the weak jaw musculature.
The respiratory system of birds is highly effective and helps meet the high oxygen demand required for flight. The avian ear is adapted to detect slight and rapid changes in pitch found in bird songs. The ossicles within green finches, blackbirds, song thrushes, and house sparrows are proportionately shorter than those found in pheasants, mallard ducks, and seabirds.
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Birds have unique neck muscles
The cervical vertebrae in birds also contribute to their advanced visual capabilities. Many birds have immobile eyes, and the flexibility of their necks allows them to move their heads and center their sharp eyesight on objects at varying distances. This stabilization of gaze is crucial during flight to safely and efficiently navigate their flight path. The neural spines of vertebrae in cervical regions 1 to 3 are heightened in intermittent bounding birds, which may represent an adaptation to counteracting oscillations during flight.
The neck muscles in birds are also involved in respiration. The active phase of respiration in birds is exhalation, which requires the contraction of their muscles of respiration. Birds lack a diaphragm, so they use their intercostal and abdominal muscles to expand and contract their thoraco-abdominal cavities, changing the volumes of their air sacs.
Additionally, the unique neck muscles in birds may be related to their dietary categories. For example, carnivorous birds require slow but forceful retraction of the neck to tear flesh from prey, while insectivorous birds need a high-velocity protraction or extension of the neck to capture escaping prey. These ecological pressures have likely influenced the construction and elongation of the cervical spine in different bird species.
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Frequently asked questions
Birds have undergone many adaptive changes for flight, including the extreme enlargement of the breast muscles. The pectoralis major, the lower muscle on the keel, is a large breast muscle that can account for a third or more of a bird's body weight. In comparison, humans have a much smaller deltoid muscle that raises the arm.
Birds have a unique set of eye muscles not found in other animals, contributing to their acute eyesight. They also have abdominal muscles that expand and contract the chest and hold the ribcage, as well as feather muscles in the skin that help adjust the feathers for flight and mating rituals.
The pectoralis and supracoracoideus muscles are responsible for producing the mechanical power required for sustained flapping flight. The pectoralis muscle lowers the wing, while the supracoracoideus raises it using a unique pulley system.
Bird muscles differ in composition and adaptation, reflecting their evolutionary path toward flight. Their striated muscles contain a respiratory pigment, myoglobin, which is more prevalent in "dark meat" than "white meat." The avian lung also differs, with nonvascular air sacs connected to the lungs, and they lack a diaphragm, relying on intercostal and abdominal muscles for respiration.
