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Birds have a light skeletal system and powerful musculature, which, along with their circulatory and respiratory systems, enable them to fly. The muscles that move the wings during flight are the pectoralis, which lowers the wing, and the supracoracoideus, which raises it. The pectoralis is the dominant muscle, and in pigeons, it represents 60% of the total wing muscle mass. The supracoracoideus is located in the angle between the keel and the plate of the sternum and along the coracoid. The keel, which is unique to birds, increases the surface area for muscle attachment.
| Characteristics | Values |
|---|---|
| Do birds have chest muscles? | Yes, birds have chest muscles. |
| Types of muscles | Cardiac (heart) muscles, smooth muscles, striated (striped) muscles, pectoral muscles, supracoracoideus muscles, intercostal muscles, abdominal muscles, feather muscles, and syringeal muscles. |
| Muscle function | Muscles that move the limbs are concentrated on the girdles and the proximal parts of the limbs. The pectoralis muscle lowers the wing, and the supracoracoideus muscle raises it. The feather muscles raise and depress feathers. The intercostal and abdominal muscles are used for respiration. |
| Muscle composition | Avian striated muscles contain a respiratory pigment, myoglobin. |
| Muscle power | Bird flight muscles contract at high frequencies and produce substantial power to support the animal's weight in the air and overcome drag. |
| Muscle adaptations | Birds have undergone many adaptive changes for flight, including the extreme enlargement of the breast muscles and the development of a unique pulley system that allows a muscle located under the wing to raise it. |
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What You'll Learn
Birds have a light but powerful musculature
The breast muscle, or pectoralis major, is a large muscle that originates along the breastbone or sternum and inserts near the head of the upper arm bone (the humerus). When the pectoralis major contracts, it brings the wing down in a downstroke motion. The supracoracoideus, a smaller muscle, raises the wing. These two muscles make up 71% of the total flight muscle mass of a pigeon. The supracoracoideus achieves a pulley-like action through a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the humerus.
The keel, or vertical sternum, increases the surface area for muscle attachment. The horizontal sternum forms a T with the keel, with the pectoralis major located at the bottom of the T. The supracoracoideus is located just above the pectoralis major and connects to the top of the humerus by means of a pulley. This pulley system is unique to birds and allows them to lift their wings using a large muscle located beneath the wing, rather than above it as in other vertebrates.
Birds also have striated (striped) muscles that move the limbs and are concentrated on the girdles and the proximal parts of the limbs. The feather muscles in the skin are smooth muscles that help in flight by adjusting the feathers, which are attached to them. These muscles also aid in mating rituals. Additionally, birds have a second set of eye muscles not found in other animals, which contribute to their acute eyesight.
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The pectoralis major is a large breast muscle
The pectoralis major is a thick, fan-shaped or triangular convergent muscle of the human chest. It is the largest muscle of the anterior chest wall and lies underneath the breast tissue. The pectoralis major arises from parts of the clavicle, sternum, and costal cartilages of the true ribs. It is also known as the "pecs", "pectoral muscle", or "chest muscle".
In birds, the pectoralis major is a large breast muscle that originates along the breastbone or sternum and inserts near the head of the upper arm bone (the humerus). When the pectoralis major contracts, it brings the arm close to the body, similar to a bird's downstroke during flight. The pectoralis major is one of two pairs of large muscles that move the wings in flight, with the other being the supracoracoideus, which raises the wing.
The pectoralis major is highly vascularised and has a rich blood supply, making it a reliable option for soft tissue reconstruction of the neck and face following trauma or tumour-related operations. It is also essential for breathing, assisting in deep or forced inspiration.
The pectoralis major is susceptible to rare injuries, particularly in high-impact contact sports such as powerlifting and weight-lifting. Tears or ruptures of the pectoralis major can result in pain, weakness, and disability.
The pectoralis major also plays a role in sensory feedback, with information processed in the superior portion of the sensory homunculus in the brain.
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The supracoracoideus muscle raises the wing
Birds have a light skeletal system and powerful musculature, which, along with their circulatory and respiratory systems, enable them to fly. The breast muscle, or pectoralis major, is a large, prominent muscle that originates along the breastbone, or sternum, and inserts near the head of the upper arm bone (the humerus). When the pectoralis major contracts, it pulls the wing down in a downstroke motion.
The supracoracoideus muscle, on the other hand, raises the wing. This muscle is located beneath the pectoralis major and is attached to the keel of the sternum. The supracoracoideus connects to the top of the humerus by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula. This tendon forms a pulley-like mechanism, which is unique to birds, that allows the supracoracoideus to lift the wing.
The development of this pulley system is one of the most dramatic adaptive changes that birds have undergone to facilitate flight. The supracoracoideus muscle is so strong that it can account for a third or more of a bird's body weight. The keel of the sternum, which is unique to birds, also increases the surface area for muscle attachment, allowing for the development of larger muscles that enable flight.
The alternating contractions of the supracoracoideus and pectoralis major allow birds to achieve the upstroke and downstroke motions necessary for flight. The supracoracoideus, in particular, with its pulley-like action, demonstrates the engineering skills that birds have evolved to enhance their flight capabilities.
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The sternum serves as an attachment site for muscles
The sternum is a flat, partially T-shaped vertical bone that forms the anterior portion of the chest wall centrally. It is divided into three segments: the manubrium, body, and xiphoid process. The body of the sternum is the longest part and provides the attachment sites for the ribs, with the second rib attaching at the sternal angle. The sternum is also known as the breastbone.
The sternum serves as an attachment site for several muscles, including the pectoralis major, which is a large breast muscle that originates along the sternum and inserts near the head of the upper arm bone (humerus). When the pectoralis major contracts, it brings the arm closer to the body, similar to a bird's downstroke. The supracoracoideus is another muscle that attaches to the keel of the sternum and connects to the top of the humerus by a pulley, allowing the wing to be raised. These two pairs of large muscles work together to move the wings in flight, with the pectoralis major lowering the wing and the supracoracoideus raising it.
The sternum also provides attachment sites for other muscles such as the transversus thoracis, which attaches to the body of the sternum and xiphoid process. The xiphoid process, or the 'tail' of the sternum, provides an attachment site for the diaphragm, a crucial muscle for respiration. Additionally, the sternum connects to the ribs via the costal cartilages, forming the anterior rib cage.
In birds, the sternum is keeled and serves as an attachment site for the muscles used in flying or swimming. Flightless birds, like ostriches, lack this keeled sternum and have denser and heavier bones compared to flying birds. The keeled structure dramatically increases the surface area for muscle attachment, allowing for the massive muscles required for flight. The coracoids are large, stout bones that connect the sternum to the shoulder, further facilitating the wing movements necessary for flight.
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Birds have advanced circulatory systems
Birds have a light skeletal system and powerful musculature, which, along with their advanced circulatory and respiratory systems, enable them to achieve very high metabolic rates and oxygen supply, which is necessary for flight.
The circulatory system of birds is more advanced than that of reptiles in several ways. Firstly, there is a complete separation between pulmonary circulation (lungs) and systemic (body) circulation, as in mammals. Secondly, the left systemic arch (aortic artery) is lost, with blood passing from the heart to the dorsal aorta via the right arch. Thirdly, the postcaval vein is directly connected with the renal portal vein that connects the kidneys with the liver. Finally, portal circulation through the kidneys is greatly reduced.
Birds have a four-chambered heart, which is relatively large compared to mammals, relative to their body size and mass. Smaller birds have larger hearts than larger birds, and among all birds, the hummingbird has the largest heart relative to its body mass, likely because hovering requires a lot of energy. The avian heart works by rapid, automatically coordinated muscular contractions that pump blood through two distinct circuits in the vascular system. The avian circulatory system also has high blood glucose levels, with about 200-400 mg/dl compared to mammals.
The avian lung differs from the type found in other land vertebrates as several pairs of nonvascular air sacs are connected with the lungs. These extend into the pneumatic parts of the skeleton. The avian lung also has a network of microscopic tubes branching off from the posterior air sacs, opening into the dorso- and ventrobronchi, as well as directly into the intrapulmonary bronchi. This network is called the "neopulmonic parabronchi" and is found in all bird species except the penguin.
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Frequently asked questions
Yes, birds have chest muscles. The pectoralis muscle is the dominant chest muscle in birds, and it is attached to the keel of the sternum. It is used to lower the wing and is essential for flight.
Birds have chest muscles that are much larger in comparison to other vertebrates. The typical vertebrate skeleton does not have enough surface area to accommodate these muscles. Birds have a keeled breastbone, which increases the surface area for muscle attachment.
The pectoralis muscle in birds is responsible for the downstroke motion of the wing, which is essential for flight. The supracoracoideus muscle, located above the pectoralis, is responsible for the upstroke motion. These muscles contract at high frequencies to produce the power needed to support the bird's weight during flight.
