The
evolution of flight has endowed birds with many physical
features in addition to wings and feathers. One of the
requirements of heavier-than-air flying machines, birds
included, is a structure that combines strength and light
weight. One way this is accomplished in birds is by the
fusion and elimination of some bones and the
"pneumatization" (hollowing) of the remaining ones. Some of
the vertebrae and some bones of the pelvic girdle of birds
are fused into a single structure, as are some finger and
leg bones -- all of which are separate in most vertebrates.
And many tail, finger, and leg bones are missing altogether.
Not only are some bones of birds, unlike ours, hollow, but
many of the hollows are connected to the respiratory system.
To keep the cylindrical walls of a bird's major wing bones
from buckling, the bones have internal strut-like
reinforcements. The pneumatization of bird
bones led to the belief that birds had skeletons that
weighed proportionately less than those of mammals. Careful
studies by H. D. Prange and his colleagues have shown this
not to be the case. More demands are placed on a bird's
skeleton than on that of a terrestrial mammal. The bird must
be able to support itself either entirely by its forelimbs
or entirely by its hindlimbs. It also requires a deep, solid
breastbone (sternum) to which the wing muscles can be
anchored. Thus, while some bones are much lighter than their
mammalian counterparts, others, especially the leg bones,
are heavier. Evolution has created in the avian skeleton a
model of parsimony, lightening where possible, adding weight
and strength where required. The results can be quite
spectacular: the skeleton of a frigatebird with a seven-foot
wingspan weighs less than the feathers covering
it! Not all birds have the same
degree of skeletal pneumatization. To decrease their
buoyancy and make diving easier, some diving birds, such as
loons and auklets, have relatively solid bones. Those birds
are generally less skillful fliers than ones with lighter
skeletons. Birds have found other ways
to lighten the load in addition to hollowing out their
bones. For instance, they keep their reproductive organs
(testes, ovaries and oviducts) tiny for most of the year,
greatly enlarging them only during the breeding
season. The respiratory system of
birds is also adapted to the demands of flight. A bird's
respiratory system is proportionately larger and much more
efficient than ours -- as might be expected, since flight is
a more demanding activity than walking or running. An
average bird devotes about one-fifth of its body volume to
its respiratory system, an average mammal only about
one-twentieth. Mammalian respiratory systems consist of
lungs that are blind sacs and of tubes that connect them to
the nose and mouth. During each breath, only some of the air
contained in the lungs is exchanged, since the lungs do not
collapse completely with each exhalation, and some "dead
air" then remains in them. In contrast, the lungs of
birds are less flexible, and relatively small, but they are
interconnected with a system of large, thin-walled air sacs
in the front (anterior) and back (posterior) portions of the
body. These, in turn, are connected with the air spaces in
the bones. Evolution has created an ingenious system that
passes the air in a one-way, two-stage flow through the
bird's lungs. A breath of inhaled air passes first into the
posterior air sacs and then, on exhalation, into the lungs.
When a second breath is inhaled into the posterior sacs, the
air from the first breath moves from shrinking lungs into
the anterior air sacs. When the second exhalation occurs,
the air from the first breath moves from the anterior air
sacs and out of the bird, while the second breath moves into
the lungs. The air thus moves in one direction through the
lungs. All birds have this one-way flow system; most have a
second two-way flow system which may make up as much as 20
percent of the lung volume. In both systems, the air is
funneled down fine tubules which interdigitate with
capillaries carrying oxygen-poor venous blood. At the
beginning of the tubules the oxygen-rich air is in close
contact with that oxygen-hungry blood; farther down the
tubules the oxygen content of air and blood are in
equilibrium. Birds' lungs are anatomically very complex
(their structure and function are only barely outlined
here), but they create a "crosscurrent circulation" of air
and blood that provides a greater capacity for the exchange
of oxygen and carbon dioxide across the thin intervening
membranes than is found in mammalian lungs. Contrary to what was once
believed, the rhythm of a bird's respiratory two-cycle pump
is not related to the beats of its wings. Flight movements
and respiratory movements are independent. The heart does
the pumping required to get oxygenated blood to the tissues
and to carry deoxygenated blood (loaded with carbon dioxide)
away from them. Because of the efficiency of the bird's
breathing apparatus, the ratio of breaths to heartbeats can
be quite low. A mammal takes about one breath for every four
and one-half heartbeats (independent of the size of the
mammal), a bird about one every six to ten heartbeats
(depending on the size of the bird). A bird's heart is large,
powerful, and of the same basic design as that of a mammal.
It is a four-chambered structure of two pumps operating side
by side. One two-chambered pump receives oxygen-rich blood
from the lungs and pumps it out to the waiting tissues. The
other pump receives oxygen-poor blood from the tissues and
pumps it into the lungs. This segregation of the two kinds
of blood (which does not occur completely in reptiles,
amphibians, and fishes) makes a bird's circulatory system,
like its respiratory system, well equipped to handle the
rigors of flight. The flight muscles of most
birds are red in color ("dark meat") because of the presence
of many fibers containing red oxygen-carrying compounds,
myoglobin and cytochrome. They are also richly supplied with
blood and are designed for sustained flight. Lighter-colored
muscles ("white meat"), with many fewer such fibers, are
found in pheasants, grouse, quail, and other gallinaceous
birds. These are also well supplied with blood, are
apparently capable of carrying a heavy work load for a short
time, but fatigue more rapidly. If a quail is flushed a few
times in a row, it will become so exhausted it will be
incapable of further flight. Finally, of course, it does
little good to be able to sustain flight or fly rapidly if
you are always crashing into things. Although birds have
found many ways to streamline, lighten, or totally eliminate
unnecessary parts (like urinary bladders), they have not
stinted on nervous systems. Birds have brains that are
proportionately much larger than those of lizards and
comparable, in fact, with those of rodents. The brain is
connected to sharp eyes, and has ample processing centers
for coordinating the information received from them. A
bird's nerves can rapidly transmit commands of the brain to
the muscles operating the wings. It is the combination of
visual acuity, quick decision making, and high-speed nerve
transmission along short nerves that permits a
Golden-crowned Sparrow to weave rapidly among the branches
of a thicket, escaping the clutches of a pursuing
Sharp-shinned Hawk. SEE: Temperature
Regulation and Behavior;
Metabolism;
Hawk-Eyed Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.