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Re: 11th specimen of Archaeopteryx



Michael Habib <MHabib@chatham.edu> wrote:

> Just as a brief reminder when talking about gravity-assisted origins of 
> flight, keep in mind that launch from a perch and launch from a
> terrestrial substrate is effectively identical in birds.  Gravitational 
> energy is, of course, useful for a gliding trajectory following an
> arboreal launch. As a result, arboreal takeoff is not objectively "easier" 
> than terrestrial launch, per se. This distinction is something
> that I expect both Tim and Don understand quite well, but it seems to have 
> been lost on some authors in the literature.


I doubt I understand it a fraction as well as you do Mike.  :-)


As far as the origin of avian flight is concerned, the implication of
typical gravity-assisted ("trees-down") scenarios is that it was the
requirements of descent that shaped the incipient wing, more so than
the kinematics of the launch.  Under "ground-up", flapping is required
to keep the animal in the air after becoming airborne, whereas under
"trees-down" the incipient wings only need to provide drag or lift.
(Disclaimer: I hate the entire "ground-up"-versus-"tree-down"
dichotomy... I'm just using these loaded terms for convenience.)


It's my understanding that birds use the hindlimbs for initial
propulsion - irrespective of whether they are launching from an
"arboreal" (elevated) or "terrestrial" (ground-level) substrate.  For
both an arboreal and terrestrial take-off, the hindlimbs provide the
thrust.  However, birds have at least initiated a complete wingbeat
cycle (unfolding, downstroke, upstroke) before the hindlimbs lose
contact with the substrate ("lift-off").  So although the wings are
not the major source of thrust for the take-off, a full upstroke is
executed after lift-off.


Modern gliding mammals also use the hindlimbs for propulsion - but
they only abduct the forelimbs prior and during the process of
becoming airborne.  Subsequent motions of the forelimb are used for
maneuvering while the glider is airborne, but not to provide thrust.


If bird ancestors were incapable of a full wingbeat (on account of the
inability to raise the humerus above the dorsum), then it would appear
that the wing was not initially evolved to provide thrust.  That is
why "passive" parachuting or gliding is often favored as the ancestral
behavior.






Cheers

Tim