Musculoskeletal system of Birds

The skeleton and muscles are part of the musculoskeletal system. The skeleton provides support for the body and organs of movement, protects the most important organs. Due to the work of the muscles, movement is carried out.

The skeleton is divided into 4 sections:

• skeleton of the head (skull);
• skeleton of the torso;
• limb girdle skeleton;
• skeleton of free limbs.

The skeleton of a bird has important features associated with flight and movement on the lower (hind) limbs.
Bones in birds are hollow, tubular or spongy, filled with air inside, which reduces the weight of the bird. The strength of the bones is given by the partitions present inside.

Head skeleton (skull)

The skull is light, consists of the cranium and jaws.
The skull is formed by fused bones.
The jaws are devoid of teeth and are covered with a horny cover – a beak.

Torso skeleton

The skeleton of the torso is formed by the spine and rib cage.
The spine is divided into five sections:

• cervical (from 11 to 25 vertebrae);
• chest (3 to 10 vertebrae);
• lumbar;
• sacral;
• tail (from 6 to 9 vertebrae ending in the coccygeal bone).

The lumbar vertebrae are completely fused with each other, with the sacral vertebrae, pelvic bones and part of the caudal vertebrae, forming a complex sacrum.
The tailbone serves as the basis for the attachment of the tail feathers.
The sternum  has a vertical outgrowth – the keel, to which the muscles that set the wings in motion are attached. Flightless birds have no keel.
The pectoral ribs are attached to the edges of the sternum.

Limb Belts Skeleton

Upper limb girdle (shoulder girdle)

• paired shoulder blades;
• paired crow bones (coracoids);
• paired collarbones (growing together, they form a “fork”, softening the movement when flapping the wings).

Hind limb girdle: formed by paired pelvic bones fused with the lumbar and sacral spine.

Free limbs skeleton

Skeleton of the upper (front) limb – wings:

• shoulder (humerus);
• forearm (ulna and radius);
• the bones of the hand (the hand has been modified: the bones of the metacarpus have grown together; the number of fingers has decreased to 3).

Hind limb skeleton:

• thigh (thigh bone);
• shin (tibia and fibula);
• bones of the foot (tarsus and phalanges of 4 fingers).

Tarsus (accrete small bones of the foot) – a device for landing in birds.
Birds walk using only their toes.

Muscle

The musculature of birds is well developed. The muscles necessary for flight are especially well developed (their weight is half the total weight of the bird!): Pectoralis major muscles (they lower the wing) and subclavian muscles (they raise the wing).

Lowering the wings in flight requires more effort from the bird than raising. Therefore, the muscles that lower the wings are much larger than the muscles that raise them.

Adaptation of the skeleton of birds to flight:

• thin bones with air cavities;
• fusion of the bones of the skull, the transformation of the jaws into a beak (absence of teeth);
• the transformation of the forelimbs into wings;
• fusion of the clavicles with the formation of a “fork”, softening the movement when flapping the wings;
• the appearance of an outgrowth on the sternum – a keel (to which the muscles that set the wings in motion are attached);
• fusion of the lumbar, sacral and tail vertebrae and the bones of the pelvic girdle (support in flight);
• the bones of the tarsus and metatarsus are fused into the tarsus (a device for “soft” landing).