Skeletal System
Skeletal System
Dr. Bruce Hunter,1 Ashley Whiteman,1 Dr. Babak Sanei,2 and Al Dam2
The avian skeleton serves many different purposes for the bird. The functions
of a skeleton include:
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a frame that supports the animal's body
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a place for muscles to attach to provide movement
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to protect the internal organs from damage
The skeleton of the bird has a number of important differences from your
skeleton. Most of these differences are necessary to make the bird as light as
possible so it can fly. Some special features of the avian skeleton include:
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Skeletal System
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fusion of many bones to increase the strength and decrease movement
of parts of the skeleton
some hollow, air-filled (pneumatised) bones to decrease the body weight
of the bird
missing bones that other vertebrates may have but birds don't need
AVIAN SKELETAL SYSTEM
rigid back bone
Avian bone
structure
Internal structure of
an avian long bone
The bones that make up the backbone (vertebral column) of birds are fused (ie.
there are no movable joints). This results in a rigid backbone necessary for flight
(try making a paper airplane and bending the back of the plane a number of
times and then see if it will fly).
The bones of the bird skeleton are extremely light-weight. This is due to a very
thin outer cortex. To maintain strength, the cortex is supported by many small
bone cross struts. In addition, many of the bird's bones are hollow ie. they have
no marrow and are filled with air. This is particularly true for bones of the head,
and the main bones of the upper wing (the humerus) and the leg (the femur).
The bird has also developed a special large modified sternum or keel bone. This
large flat bone protects the internal organs such as the heart and lungs and also
supplies a large flat surface for the attachment of the major muscles used for
flight.
Human body builders spend a lot of time building up their "pecs". The pectoral
muscles on a bird are the main muscles responsible for flight because they lift
the wing and are responsible for the
down-stroke of the wing during flight.
Great Horned Owl Skull
thin skull
bony eye sockets
The bones of the skull are fused
together, flat and very thin. Many birds
that have large eyes, such as hawks,
eagles and owls have reinforced bony
eye sockets to protect the large eyeballs.
These birds have bone plates within the
eyeball called "bony sclera" to allow it to
keep its shape.
The bones of the upper and lower jaws
support the beak structure. Birds do not
have teeth, so their jaw bones do not
need to be massive.
light jaw bones
Avian skull
Labelled avian skeleton
FACTSHEET 5.2
Skeletal System
Each bird species has its own special skeletal adaptations. For example:
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FACTSHEET 5.2
March 2008
Small little hummingbirds have only 14 bones in their neck, while
long necked swans have 25.
The bones of the neck in an owl are specially formed so owls can
turn their head 270 degrees to look or reach behind them.
Although all birds that fly have the bones making up their backbones
fused to create a rigid structure for flight, the penguin has mobile
vertebrae that allows great flexibility for swimming in the water.
Flying birds have a well-developed keel bone but the flightless
ostrich and emu have no keel at all (the term ratite means flat or raftlike keel).
As we have selected for rapid growth in our commercial birds, we have
inadvertently created a large number of disease problems associated
with the skeleton. Broiler chickens now reach a target weight of 2.4 kg in
under 38 days. Heavy body weight on an immature skeleton creates
many stresses that often end up in bone deformities. Many of these
conditions including twisted legs (valgus/varus deformities) in broiler
chickens and other poultry species or shaky leg in heavy turkeys are a
result of fast growth and heavy weight placed on an immature skeleton.
Proper nutrition is critical in helping the bones develop normally under
these stresses. Nutritional diseases, particularly calcium/phosphorus or
vitamin D3 imbalances are very common in any species raised on a
home-made diet. Several of these skeletal diseases are discussed in
greater detail in the disease fact sheets.
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