THE ANATOMY OF FLIGHT
How birds fly is a fascinating study of how evolution molds living
things to suit the demands of survival. I have summarized the
major modifications below.
Enlarged Breast Muscle & Sternum
To accomplish flight the most obvious modification is in the breast
muscles which make up nearly one third of a birdís body weight.
These muscles attach to an enlarged sternum (which looks like the
keel of a sailboat protruding from the chest). The other end of the
muscles attach to the stout humerus wing bone.
Unique Pulley System
To raise their wings up while flapping, birds do not use shoulder
muscles, but instead have a unique pulley system that allows
muscles attached below the wings (below the birdís center of gravity)
to pull the wings up. The long tendon of the supracoracoideus
muscle loops up and over the shoulder bone to raise the wing when
the muscle contracts downward.
Wishbone is Spring-loaded
In the chest of a bird is the V-shaped Furcula bone (also known as
the wishbone). As the huge breast muscles pull the wings downward,
this bone is simultaneously pulled wider so that it acts as a spring-like
mechanism on the rebounding upstroke.
Muscles Eliminated to Reduce Weight
In mammals and reptiles, flexibility is achieved through many bones
interconnected by muscles. In birds many of these interconnections
have been eliminated through fusion of bones into rigid structures
that not only reduce weight, but provide rigid strength under the stress
Further Reduction in Weight
Another means of weight reduction has been elimination of the heavy
muscles in the head required to chew food. In birds this function is
accomplished in the gizzard located back toward the birdís center of
Flexible Necks Compensate
Since the front limbs of birds have been totally modified for flight,
making them useless for anything else (such as manipulating food or
grooming), the structure of the neck has increased its flexibility and
mobility. Many birds hold their necks in an S-shaped position and
are capable of stretching out and reaching all of their feathers for
preening. In fact a bird must be able to reach the base of its tail
where it can gather oil from the uropygial gland to smooth onto its
Flight burns up enormous amounts of energy. Birds consequently
have very efficient digestive systems. To stay as light as possible,
they donít have the luxury of storing energy. Only during migration
do birds begin to stock up.
On average a bird's heart is forty percent larger than that of a
mammalís. Flight requires incredible circulation.
Air Sacs Allows Internal Cooling & Efficient Respiration
Since birds have no sweat glands and are covered with insulating
feathers, their means of cooling takes place internally. The prodigious
activity of flight creates lots of heat. Air that is inhaled, passes through
a series of air sacs arranged throughout the bird's body to carry this