The avian skeleton is much lighter than that of mammals, in fact, a large part of their bones contain air (pneumatization) instead of bone marrow. These cavities are communicated with the respiratory system and act to decrease weight, so flying is easier.
Bones which are not pneumatised include most vertebrae, and those distal to the humerus and pelvis. Avian bones are richer in inorganic substances (calcium phosphate). Long bones have a very thin cortex and the medullary cavity contains a network of trabeculae, which increases the strength of the bone.
These factors mean that avian bones are harder, but at the same time more fragile and less elastic than those of mammals. This means that when fractured they splinter easily, but it is impossible to use plates or intramedullary screws to aid healing as they destroy the internal structure. Making external fixators is the most appropriate treatment for fractures.
Characteristic features of the avian skull include the vaulted cranium, the large bony orbits, separated by the interorbital septum, and the beak-sheaped pyramidal face.
The boundaries between different bones is hard to define because of the conversion of sutures to synostosis, a few months after hatching. The upper jaw is formed by the nasal bone, maxilla and premaxilla
and the lower jaw by five little bones that fuse forming the mandible.
Notable features in the avian skull include the presence of a single occipital condyle and the so named quadrate bone.
The quadrate bone connects the mandible to
the skull (temporal bone). Quadrate bones are the most important component
of the maxillopalatine apparatus. These enable the upper and lower jaw to
move simultaneously, while the quadrate bone turns, increasing the size of
In psittacines (parrots) the craniofacial joint is synovial and the range of movement of upper and lower jaws is wider and stronger.
Most of the skull bones are pneumatised. These bones communicate with the nasal and tympanic cavities, so that the weight of the head is reduced and this helps flying. The absence of teeth also contributes to this.
Skeleton of trunk (vertebrae, ribs and sternum)
The avian rachis consists of cervical (C), thoracic (T), sacral (LS) and coccygeal (Cd) vertebrae.
The vertebral formula varies between different species, and in comparison to mammals there are a larger number of cervical vertebrae. Generally the formula is: C14, T7, LS14, Cd6. The number of cervical vertebrae is variable (13 to 25). These vertebrae have got prominent transverse processes for muscle attachment. The “S” shaped neck is needed in some species, in order to protect the encephalus from the shake produced during take off and landing.
The small ring-shaped atlas articulates with
a single occipital condyle, so that the atlanto-occipital joint is very mobile,
which allows the head and beak to move in different directions. The number
of thoracic vertebrae is fewer than in mammals (5 to 7),
and many of them fuse forming the notarium bone. The first vertebra following
the notarium is the only mobile vertebra of the rachis. Trauma
can cause this vertebra to move ventrally damaging the spinal cord, causing
the pathology known as “kinky back”. The last two thoracic vertebrae
fuse with the lumbar, sacral and first two coccygeal vertebrae, forming the
synsacrum bone, which finally fuses with the ilium.
The notarium and synsacrum give strength to the rachis. The first coccygeal vertebrae allows tail movement, while the last 4-6 coccygeal vertebrae fuse, forming the pygostyle .
Ribs are found on both sides of the rachis, the first two or three are “false ribs”, while the rest are “true ribs”. The bony ribs are divided into two parts, the vertebral (proximal) part, and the sternal part. The ribs in the middle of the thorax have uncinate processes,
which rise in a caudodorsal direction and
lie against the lateral surface of the next rib. This allows the thorax to
support the ventral movement of the wing while flying.
The sternum is a large bone with many processes, notches and openings.
There are pneumatic openings on its dorsal surface which communicate with the clavicular air sac.
The spine of the sternum on the ventral surface (keel) is where the pectoral muscles insert. This landmark is well developed in flying birds (carinates).
Conversely, in birds which do not fly (ratites) the ventral surface of the sternum is flat.
The subcutaneous position of the sternal keel is useful to obtain bone marrow samples in large cage kept birds, but also makes them vulnerable to deformities and injuries of the sternum when they rest on inadequate perches. The caudal extremity of the sternum is cartilagenous in young birds and ossifies with age. Therefore, its flexibility can be used as an indicator of the age of the bird.
Skeleton of the pectoral limb:
The transformation of the pectoral limbs into wings has caused important changes, which can be summarised by the following:
There is a complete appendicular skeleton formed by three bones: coracoid, clavicle and scapula.
The thoracic skeleton has fewer bones than that in mammals and the humerus is pneumatized.
In the proximal thoracic skeleton the coracoid bone is well developed, and connected to the sternum by the pectoral limb. It holds the wing away from the sternum during flight, and prevents the thorax from collapsing during the downstroke of the wing. The two clavicles fuse ventrally to form the furcula; which joints the sternum and coracoid bone by the sternocoracoclavicular membrane (membrana sternocoracoclavicularis). The furcula acts as a spring holding the shoulder joint at the right distance during flight.
The clavicle may not be present or may be reduced in some species of parrot. The scapula is narrow and curved, lying laterally and dorsally to the thorax, and attached to it by muscles and ligaments. Between the three proximal bones (coracoids, clavicle and scapula) a canal is formed (the triosseal canal), which is a passage for the tendon of the deep pectoral muscle.
The avian humerus is similar to mammals,
but pneumatized. The pneumatic opening is situated on the proximal end, where
two tubercles are seen (dorsal and ventral) for muscular insertion.
While the wing is folded, the humerus lies against the thorax and parallel to the scapula.
The ulna is more developed than the radius and both are bowed along their length, so they are protected against bending forces. The distal epyphisis of the ulna can be used for intramedullary administration of substances.
There is a great reduction in the number
of carpal bones, compared to mammals. The proximal row consists
of a radial carpal bone and an ulnar carpal bone, while the distal row fuses
with the metacarpus forming the carpometacarpus. The three digits are the
major digit, with two phalanges, the minor digit, with one phalanx and the
alula digit, with two phalanges.
Attached to the skin over the carpometacarpus and the phalanges of the major and minor digits there are the primary flight feathers. The alula digit forms the skeleton of the alula or bastard wing.
In chicks amputation or surgical pinioning of the wing can be carried out by removing the carpometacarpus, so the bird is unable to fly.
Skeleton of the pelvic limb:
The pelvic limbs are involved in locomotion, both on land and in the water.
The proximal skeleton consists of three bones, as in mammals: ilium, ischium and pubis, which form the girdle.
The two girdles do not fuse ventrally (and therefore the pelvic symphisis is not present), except in a very few species,
but ilium fuses with the synsacrum. The ischium
is proportionally larger than in mammals, forming a large part of the lateral
wall of the pelvis. The pubis is long and thin, palpable through the skin
and its flexibility indicates the age of the bird.
The distal end of the femur slopes craniolaterally bringing a large part of the hind limb close to the centre of gravity of the body. A patella is present and fibula is reduced to a thin bone,
the tibia is fused with the proximal row of the tarsal bones to form the tibiotarsus.
The femur and tibiotarsus are very rich in
bone marrow, unlike other long bones.
The foot skeleton is formed from metatarsals II, III and IV, which fuse with the distal tarsal bones forming the tarsometatarsus.
There is a small metatarsal remnant which is attached to the tarsometatarsus by ligaments.
Thus in the bird there are four digits in the foot (I to IV), formed by either two, three, four or five phalanges. The most distal phalanx forms the bony core of the claw.