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 bird’s 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 bird’s 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
of flight. 

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 bird’s center of
gravity.

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
feathers.

Fast Digestion

Flight burns up enormous amounts of energy.  Birds consequently
have very efficient digestive systems.   To stay as light as possible,
they don’t have the luxury of storing energy.  Only during migration
do birds begin to stock up.

Large Hearts

On average a bird's heart is forty percent larger than that of a
mammal’s.  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
heat outward.