Lecture 13, Feb 15, 2011
Bird flight
Gas exchange with bird lungs
Check Cornell Laboratory of Ornithology website for good information
The aerofoils of birds: the wing is an aerofoil in its shape in transverse section and generates lift (in
conjunction with a power and recovery stroke)
Bernoulli’s Principle: fluid moving at
higher speed develops a lower pressure.
Blowing above a ribbon causes it to lift
illustrating Bernoulli’s principle.
A bird wing in section is
shaped like an aerofoil: flat
below, arched above, thick in
front then tapering rearward
to thin behind
Canoe compared to rowboat:
minimal drag because of
tapered stern.
Tapered aerofoil shape plus
streamlining effect of feathers
reduces the turbulence that
promotes drag. Depending on
the angle of attack of the wing,
air flows over the upper surface
smoothly and tends not to break
away to the rear, keeping drag
low and lift relatively high.
Keratin* is the material of feathers. The
long tapered central support of a bird’s
feather is the rachis; it separates
interlocking barbs (and barbules) on each
side which constitute a sheet called a
vane. The adaptiveness of this may have
to do with preening (a groomable
surface). Typical body-contour feathers
have symmetrical vanes (or nearly).
Asymmetry occurs in primary wing
feathers: the rachis of the primaries is
shifted toward the leading edge of the
feather: so one edge is thicker, stiffer and
narrower, rather than at the middle of the
feather. Asymmetry is thus found in
feathers that have their leading edges in
close contact with the flow of air in flight.
The asymmetry gives each feather an
airfoil transverse section. Primary feather:
quill, rachis, vane, barb
* [from Wikkipedia] Keratin refers to a family of fibrous structural proteins. Keratin is the key structural material making up the outer
layer of human skin. It is also the key structural component of hair and nails [and the feathers of birds]. Keratin monomers assemble into
bundles to form intermediate filaments, which are tough and insoluble and form strong unmineralized tissues found in [tetrapods]. The
only other [material] known to approximate the toughness of keratinized tissue is [the polysaccharide] chitin.
Vane asymmetry primary feather functions as a self-contained aerofoil, See Feduccia, A. & Tordoff, H.B. 1979. Feathers of
Archaeopteryx: asymmetric vanes indicate aerodynamic function. Science 203: 1021-1022
Selective forces acting upon
feathers are many
Birds that have evolved to swim are moving in a much denser
fluid than air: they will show modifications in feather structure
[what might be the feather modifications of a penguin flying in
water contrasted with those of birds flying in air? They still
require feathers to streamline and reduce the turbulence that
promotes drag.
Some feathers are not naturally selected: they are
the product of sexual (not natural) selection.
Females choose the best male based upon the
stimulation of his plumage; female choices become
the basis of reproductive success. This kind of
selection can produce a feather that does not
streamline and actually promotes drag.
Foramen triosseum
[‘3-bone opening’] is
an opening created
by the joining
together of three
bones: humerus,
scapula and coracoid
Muscles that power
bird flight
Coracoid bone is part of wishbone/furcula
Sam Mozley
Through the foramen triosseum runs the tendon of the supracoracoideus muscle; it inserts on the upper surface of the humerus and
works in a pulley fashion through the foramen to lift the humerus (and the wing), pivoting the proximal end of the humerus against
the scapula and coracoid. These two powering flight muscles (elevator and depressor) lie one on top of the other (the depressor is
‘superficial’, lies above); both have fibres fastened to the keel at one end and to the humerus at the other. Both muscles constitute
third-class levers with a poor force advantage and a reasonably good distance advantage.
Costal suction pump
intercostal muscles run between
ribs and contract to move
ribs forward and down during
inspiration: sternum moves
forward and down
volume of thoracic cavity greatly
reduced internal pressure
air inrush
Sternum moved down (and up) by supracoracoideus and pectoralis major operating
during flight: flight directly linked to ventilation
Bird lung below with air sacs
shown to the right
Syrinx located at junction
of trachea and bronchi is
an organ for sound
Lungs of human occupy about 5% body volume. Lungs of bird
only occupy 2% of body volume. But lungs plus air sacs of a
duck occupy about 20% of its body volume. Nevertheless bird
air sacs are not involved in gas exchange: they are not
vacularized (no blood vessels): but they are involved in air
circulation. Air sacs are reconnected to the mesobronchi by
recurrent bronchi.
There are 9 air sacs: an anterior group: interclavicular
(1), cervical (2), anterior thoracic (2); posterior group:
abdominal (2), posterior thoracic (2). Unpaired
interclavicular air sac in anterior midline sends
diverticulae into some of larger bones (sternum
pectoral girdle): pneumatic bones: this serves to lighten
Tiny air capillaries within the lung proper the bird for flight.
are site of gas exchange.
Trachea forks (at syrinx) into two
primary bronchi, one going to each
lung; as each primary bronchus passes
through the (right or left) lung its name
changes to mesobronchus. At the
mesobronchus’ anterior end arise a
group of secondary bronchi, this also at
its posterior end: these anterior and
posterior secondary bronchi are
connected by parabronchi.
Diagram to right is simplified bird lung, ‘it
‘groups’ anterior and posterior air sacs in
order to more easily visualize the air
circulation. The lungs cannot change
their volume, but the air-sacs do. Two
cycles of inspiration and expiration
(powered by the muscles of flight ,
including the intercostal muscles
between the ribs of the thorax) are
required for one breath to make its way
through the system, in and out again; it is
a true circulation and not a tidal system
such as in mammals. Follow one ‘breath’
through this system:
The posterior and anterior air sacs expand on inhalation and constrict
on exhalation (inspiration and expiration are alternative terms), this
being caused by the motions of the sternum and rib cage during flight.
On inhalation (1) all sacs expand and new air (a ‘breath’) moves [mostly]
directly into the posterior air sacs along the mesobronchus. At the same
time the expanding anterior air sacs draw air forward from the
parabronchi. On exhalation (1) all the sacs are constricted again and this
pushes the air of the posterior sacs (the breath) forward into the
parabronchi. Now the flight motion brings about air sac expansion,
inhalation (2); all sacs expand and the expanding anterior sacs draw the
air into them (the breath we are following) forward from the
parabronchi; finally we have exhalation (2) and the breath moves from
the anterior sacs back to the outside.
Mammalian lungs expand and contract during each cycle
of inspiration and expiration: the ventilatory cycle. During a bird’s
ventilatory cycle the “air sacs suck and push gases through the rigid
tubing of the lungs”
Old note
Even older
Some references re bird flight
Ji, Q., Norell, M.A., Gao, K-Q., Ji, S-A., and Ren, D. (2001) The distribution of
integumentary structures in a feathered dinosaur. Nature, 410, 1084-1088 (April
Norell, M., Ji, Q., Gao, K., Yuan, C., Zhao, Y., and Wang, L. (2002) 'Modern' feathers
on a non-avian dinosaur. Nature, 416, 36-37 (Mar 7).
Prum, R.O. (1999) Development and evolutionary origin of feathers. Journal of
Experimental Zoology (Molecular and Developmental Evolution), 285(4), 291-306.
Sues, H-D. (2001) Ruffling feathers. Nature, 410, 1036-1037 (Apr 26).
Xu, X., Zhou, Z-h., and Prum, R.O. (2001) Branched integumental structures in
Sinornithosaurus and the origin of feathers. Nature, 410, 200-204 (Mar 8).
Yu, M., Wu, P., Widelitz, R.B., and Chuong, C-H. (2002) The morphogenesis of
feathers. Nature, 420, 308-312 (Nov 21).

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