Lecture 13, Feb 15, 2011

Bird flightFeathers

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.

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

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.

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.

DrawingSam Mozley

Muscles that power bird flight

Foramen triosseum[‘3-bone opening’] is an opening created by the joining together of three bones: humerus, scapula and coracoid

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.

Coracoid bone is part of wishbone/furcula

Costal suction pumpintercostal muscles run between

ribs and contract to moveribs forward and down during inspiration: sternum moves

forward and downvolume of thoracic cavity greatly

increasedreduced internal pressure

air inrush

Sternum moved down (and up) by supracoracoideus and pectoralis major operatingduring flight: flight directly linked to ventilation

Bird lung below with air sacs shown to the right

Tiny air capillaries within the lung properare site of gas exchange.

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 the bird for flight.

Syrinx located at junction of trachea and bronchi is an organ for sound generation

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 note

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 26).

• 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).