Hi John,
Thanks for bringing the topic of flight energetics to our attention. I have some
knowledge of avian energetics, physiology and anatomy, so I can offer a little
bit of
help, but there are no doubt others out there who can add more information.
As far as I am aware, soaring birds do not have a means of locking their
?shoulders? in
flight. There is very little weight loading carried by the pectoral muscles,
because of
the structure of the wing and the light weight of a bird. Unlike mammals, birds
have
hollow bones with internal cross-strutting, thus reducing the overall weight of
the
wings and other parts of the body. The air-stream passing underneath the wing
as the
bird flies through the air also pushes the wings upwards compared with the
air-stream
that passes over the top of the wing, and this is assisted further by rising
thermals.
So the pressure on the pectoral muscles in supporting the outspread wings is
not as
great as one would first imagine.
Unlike a fixed-winged plane or glider, a bird has the ability to alter the area
and
shape of its wings and their position with respect to the body. Such an ability
assists
the bird to manoeuver in flight and change the aerodynamic qualities of its
wings to
meet the birds flight requirements under different conditions. Furthermore, the
wing is
not a solid structure like a plane?s wing, but allows the flow of air through
and
between the feathers, further reducing the drag loading on the pectoral muscles.
It is not difficult to maintain circulation of oxygenated blood through the
pectoral
muscles and other flight muscles while a bird is soaring. Firstly, birds have
very
powerful hearts that beat very quickly and thus are very efficient in pumping
blood to
all parts of the body, including into the pectoral muscles and the wings.
Secondly, the
flight muscles are very well vascularised, further facilitating the flow of
blood and
oxygen to the tissues. Although a bird may soar in the air for hours, it will
occasionally flap its wings; this physical movement also helps to direct the
flow of
blood to and from the flight muscles.
The pectoral muscle of soaring birds also consists of two types of muscle fibre,
voluntary skeletal muscle for power of contraction and deeper involuntary
(tonus)
skeletal muscle for sustained use.
Birds also have a very efficient respiratory system to increase the uptake of
oxygen and
removal of carbon dioxide from the body tissues. The lungs also extend into
air-sacs
that extend to different parts of the body, and occupy about 20% of the volume
of a
bird?s body. Such a system allows a bird to fly at high altitudes where oxygen
levels
are low and it also allows oxygen to reach body tissues quickly through rapid
diffusion
into the circulatory system.
You are quite right in saying that wing flapping assists respiration in birds,
but it is
not necessary unless an increased rate of gaseous exchange is required. The
upper
surfaces of the lungs adhere to the ribs, and the lower surfaces adhere to a
membrane
lining the body wall. Quiet respiratory movements when a bird is resting on
land or is
soaring in the air are produced by the intercostal (inspiratory) and abdominal
(expiratory) muscles acting upon the thoracic and abdominal cavities so as to
enlarge
and contract the thorax, drawing air in and out of the air sacs, through the
lungs. When
a bird flaps its wings in flight, the ventilation of the respiratory system is
provided
by the pectoral muscles which move the sternum towards and away from the
vertebral
column.
It can be quite costly in terms of energy for birds to soar for long periods of
time.
However, soaring birds (e.g. raptors, albatrosses, swifts etc.) usually have
protein-rich diets (e.g. fish, terrestrial vertebrates, insects), so under
normal
situations this form of foraging is cost-effective. Most soaring birds also
usually
travel long distances in search of food (e.g. albatrosses can cover thousands of
kilometres of the Southern Ocean in the space of several days) and can only do
this by
soaring because it is too energy-expensive to actively fly these distances.
It is not uncommon for Wedge-tailed Eagles to be seen soaring at several
thousand feet.
At these heights they are usually looking out for intruders in their territorial
air-space.
Dr Stephen Ambrose
Research and Conservation Manager.
******************************************************
Birds Australia (formerly Royal Australasian Ornithologists Union).
Australian Bird Research Centre,
415 Riversdale Road,
Hawthorn East,
VIC 3123.
Tel: (03) 9882 2622.
Fax: (03) 9882 2677.
Email: S.Ambrose <> (at work)
(at home)
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