All vertebrate animals have skeletons. A skeleton allows an animal to stand and protects its internal organs and tissues. The avian skeletal system looks similar to that of mammals but must accommodate a bird's need to be light enough to fly while having necessary body support. Consequently, the skeleton of a bird includes some unique features.
The bones of birds are lighter in weight than those of mammals. Some of the bones are hollow and actually act as part of the avian respiratory system. These bones, called pneumatic bones, include the skull, humerus, clavicle, keel, pelvic girdle, and lumbar and sacral vertebrae.
Other important bones in the avian skeleton are the medullary bones. These bones include the tibia, femur, pubic bone, ribs, ulna, toe bones, and scapula. Medullary bones are an important source of calcium when hens are laying eggs. Eggshells primarily are made of calcium, and a hen's body mobilizes 47 percent of its body calcium to make an eggshell. When in production, a commercial laying hen cannot obtain enough dietary calcium to allow for daily egg production. Without medullary bones to draw calcium from, the hen would produce eggs with very thin and weak shells.
Although important differences exist between the skeletons of birds and other animals, several similarities are present as well. In general, birds have the same skeletal structure as many other animals, including humans (as shown in Figures 1 and 2).
Fig. 1. Comparison of chicken (left) and human (right) arm bones. Source: Public domain.
Fig. 2. Comparison of chicken (left) and human (right) leg bones. Source: Public domain.
The common joints of the arms of chickens and humans are easily identifiable.
Both the human leg and chicken leg have a femur, a fibula, and a tibia. In a chicken, the femur holds the thigh meat, and the fibula/tibia combination holds the meat of the drumstick. The metatarsus of a chicken is known as the shank, and the chicken walks on its toes. A comparison of the leg joints of chickens and humans is not quite as obvious as a comparison of the arm joints.
An animation showing the Biomechanics of avian flight is available online and explains the skeletal adaptions that allow for flights.