Dominic Stone
Birds have undergone many adaptations to enable flight, including the development of a unique pulley system that allows a muscle located under the wing to raise it. The pectoralis muscle is the dominant avian flight muscle, and in pigeons, it represents 60% of total wing muscle mass. Birds also have supracoracoideus muscles, which are about one-fifth the size of the pectoralis and are the primary wing elevator active during the upstroke. The pectoralis major, the lower muscle on the keel, is shown contracting, pulling the wing down. The extreme enlargement of the breast muscles and the skeletal modifications that accommodate them are among the most dramatic adaptive changes birds have undergone for flight.
Explore related products
What You'll Learn
The pectoralis muscle is the dominant flight muscle in birds
The pectoralis muscle is one of two primary flight muscles in birds, the other being the supracoracoideus muscle. The supracoracoideus is much smaller, weighing about one-fifth as much as the pectoralis, and is responsible for raising the wings, generating the upstroke force. The two muscles work together to enable birds to fly.
The pectoralis muscle is significantly larger in birds than in other flying species, powering the downstroke of the wings during flight. The size of the pectoralis muscle is particularly notable in hummingbirds, the smallest avian species, allowing them to sustain hovering. In pigeons, the pectoralis muscle represents 60% of total wing muscle mass. Measurements of pectoralis mechanical power output and wingbeat frequency have been studied in black-billed magpies, cockatiels, and ring-necked doves across a range of flight speeds in a wind tunnel.
The extreme enlargement of the breast muscles in birds is one of the most dramatic adaptive changes for flight. The addition of a vertical keel to the sternum provides an increased surface area for muscle attachment, accommodating the large pectoralis muscle. The keel forms a T-shape with the horizontal sternum, with the pectoralis major located on the lower part of the keel. This unique structure enables the muscle to function by shortening, pulling the wing down with significant force.
Caterpillars' Muscles: The Surprising Number and What It Means
You may want to see also
Explore related products
The supracoracoideus muscle is the primary wing elevator
Birds have undergone many adaptive changes to enable flight. One of the most significant adaptations is the extreme enlargement of the breast muscles, specifically the pectoralis and supracoracoideus muscles, which power the downstroke and upstroke of the wings. The pectoralis muscle is the dominant avian flight muscle, representing around 60% of total wing muscle mass in pigeons. It is attached to a vertical keel on the sternum, which increases the surface area for muscle attachment. This powerful muscle depresses the wings at the shoulder, producing substantial mechanical work during the downstroke and pronating the wing.
The supracoracoideus muscle, on the other hand, is much smaller, about one-fifth the size of the pectoralis. It originates from the keel of the sternum, located deep beneath the pectoralis muscle. The supracoracoideus is the primary elevator of the wing, active during the upstroke, particularly at slow to moderate speeds and during hovering. It achieves this through a long tendon that passes over the shoulder, acting as a pulley to elevate the wing and supinate the wing. The contractile portion of the supracoracoideus arises from the ventral sternum, with its tendon of insertion coursing above the glenohumeral joint to insert on the dorsal surface of the humerus. This unique morphology enables the supracoracoideus to impart a high-velocity rotation to the humerus, rapidly elevating the distal wing.
The combination of the pectoralis and supracoracoideus muscles, along with other smaller extrinsic and intrinsic wing muscles, allows birds to achieve the power and control necessary for flight. These muscles work together to modulate wing orientation and control wing shape, contributing to adjustments in the wing's performance as an aerofoil. The function of these muscles is tailored to meet the mechanical power requirements of flapping flight, with the pectoralis muscle contracting at high frequencies to produce the aerodynamic power needed to support the bird's weight in the air and overcome drag.
The evolution of the supracoracoideus muscle was an important step in the development of powered flight in birds. Late Jurassic and Early Cretaceous birds lacked a morphologically derived supracoracoideus, limiting their ability to achieve a high-velocity recovery stroke. The subsequent evolution of this muscle enabled birds to impart a large rotational force to the humerus, enhancing the upstroke and reducing air resistance during the recovery stroke.
Understanding Muscle Knots: Causes and Formation
You may want to see also
Explore related products
The development of a unique pulley system
Birds have developed a unique pulley system that allows a muscle located under the wing to raise it. This is an adaptation to the anatomy of the vertebrate shoulder, which does not provide a site for attaching a larger arm-raising muscle. The pectoralis major muscle is large, and the deltoid is small, so a bird can bring its wing down with much greater force than it can lift it.
To address this, birds evolved a vertical keel on the sternum, which dramatically increases the surface area for muscle attachment. The horizontal sternum forms a T with the vertical keel. The pectoralis major, the lower muscle on the keel, attaches to the humerus and pulls the wing down. As muscles can only function by shortening, a muscle must be located above the wing to raise it.
The supracoracoideus muscle is about one-fifth the size of the pectoralis and is the primary muscle active during the upstroke, especially at slow to moderate speeds and during hovering. At faster flight speeds, wing elevation is likely produced passively by aerodynamic forces acting on the wings, which remain extended during the upstroke to maintain lift. Smaller extrinsic and intrinsic wing muscles assist in modulating wing orientation and controlling wing shape.
The pectoralis muscle in birds contains predominantly fast-twitch fibres and has been studied for its typical myofibril and sarcomeric structures and myofilament protein contents. The pectoralis represents 60% of total wing muscle mass in pigeons and is the dominant avian flight muscle.
Muscle Weakening: How Quickly Does Strength Fade?
You may want to see also
Explore related products
The function of the coracoid bones
The coracoid is a stout, hook-like bone that connects the sternum with the shoulder. It is also known as the "surgeon's lighthouse" because it serves as a landmark to avoid neurovascular damage. The coracoid is the most commonly fractured bone in the shoulder. Its highly protected position beneath the pectorals makes it relatively invulnerable to injury. However, it is susceptible to fracture when a sudden downward force is exerted on the upper wing surface.
The coracoid is a crucial structure in the pectoral girdle of birds, providing a rigid and stable brace necessary for flight. It articulates with the scapula and the furcula (wishbone) near the shoulder joint, acting as a strut between the sternum and the shoulder joint. The coracoid widens ventrally and articulates with the body of the sternum, providing stability and support for the wing muscles.
The coracoid process is a specific structure associated with the coracoid bone. It is a small, hook-like projection that extends from the lateral edge of the scapula (shoulder blade). This process, together with the acromion, helps to stabilize the shoulder joint. It serves as an attachment site for several muscles, including the pectoralis minor, coracobrachialis, and biceps brachii. Additionally, the coracoid process plays a role in shaping the curve over the glenoid by connecting the clavicle to the scapula.
The coracoid bone and its associated processes are not unique to birds. They are also found in other animals, including dinosaurs, reptiles, monotremes, and even some extinct therapsids. In dinosaurs, the coracoid and scapula were the main bones of the pectoral girdle, directly articulating with the clavicle. In monotremes, the coracoid bone is similar to that of reptiles, while also possessing a separate coracoid process like other mammals.
Prions: Hiding in Regular Muscle?
You may want to see also
Explore related products
The evolution of bird muscles
The pectoralis muscle, also known as the pectoralis major, is the dominant flight muscle in birds, contributing to up to 60% of total wing muscle mass in pigeons. It is responsible for depressing the wings at the shoulder, generating the force required for the downstroke during flight. The supracoracoideus muscle, on the other hand, is smaller and acts as the primary wing elevator during the upstroke, particularly at slow to moderate speeds and during hovering.
To accommodate the powerful pectoralis muscle, birds have undergone significant skeletal modifications. One notable adaptation is the addition of a vertical keel to the sternum, which increases the surface area for muscle attachment. This keel forms a T-shape with the horizontal sternum, providing a sturdy base for the pectoralis muscle to insert and generate force. The supracoracoideus muscle lies in the angle between the keel and the sternum, assisting in the upstroke motion.
Furthermore, the evolution of bird muscles has resulted in the presence of striated muscles, which are responsible for moving the limbs. These muscles are concentrated on the girdles and proximal parts of the limbs. The feather muscles, a type of smooth muscle in the skin, play a role in raising and depressing feathers, contributing to aerodynamics and temperature regulation. Overall, the evolution of bird muscles has been shaped by the demands of flight, leading to unique adaptations that enable birds to soar through the skies with ease.
The Heart's Muscle: Understanding Cardiac Function
You may want to see also
Frequently asked questions
Yes, bird wings have muscles. The pectoralis muscle is the dominant muscle in bird wings and can represent up to 60% of the total wing muscle mass.
The pectoralis muscle is the major muscle that drives the downstroke of the wings during flight.
The supracoracoideus muscle is the primary muscle active during the upstroke of the wings, particularly at slow to moderate speeds and during hovering. Smaller extrinsic and intrinsic wing muscles also assist in modulating wing orientation and controlling wing shape.
Flying birds have larger pectoralis and supracoracoideus muscles compared to other flying species, which are essential for powering the downstroke and upstroke of the wings during sustained flight.
![The Structure of Wing-Feathers / by E. Mascha 1905 [Leather Bound]](https://m.media-amazon.com/images/I/617DLHXyzlL._AC_UL320_.jpg)
