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Re: Science feather strength debate



> A symmetrical feather can therefore only operate as a useful lifting 
> surface if both rachis and socket are able to resist any torsional 
> forces generated,
...
> theories both consider flight to have been the selective
> force driving the evolution of the wing, both require that
> any feathers orientated across the airflow be asymme-
> trical as soon as they appear."

Again, I think there is a "logical leap" being made, that isn't necessarily 
true.
It is relatively hard to resist torsional forces with a single attachment 
point, but I would think that if feathers are overlapping, that a feather could 
be supported not only by its rachis, but by the rachis of the feather it 
overlaps with. In this case, in order for the overlap to be useful at resisting 
the torsion, the rear of the leading feather must rest on top of the feather 
behind it, which is the opposite of the case in birds today I believe, but I 
presume this is evolutionarily easy to change.
Is the preservation detailed enough to tell if this was the case in anchiornis?
- mechanically, the way modern bird feathers overlap, makes sense in light of 
their asymetrical feathers, as it allows the trailing edge of a feather to be 
supported by the feather behind it, thus the trailing edge barbs coming from 
the central shaft don't need to be as stiff (would the weight saving be 
significant using this overlap vs the other overlap scheme, or might it allow 
for easier folding if the broad trailing portion of the feather is a little 
more flexible?)

While mechanically, asymmetrical feathers are much better for aerodynamic 
functions, I don't think it is a requirement that feathers oriented across the 
airflow be asymmetric as soon as they appear.
However, if I were to see tip feathers widely spaced like in a vulture:
http://www.calliebowdish.com/Birds/TurkeyVultureShaverLake090606_7491.jpg
- and they were asymmetric, then an aerodynamic function seems much less likely.



--- On Fri, 11/5/10, Tim Williams <tijawi@gmail.com> wrote:

> From: Tim Williams <tijawi@gmail.

> Subject: Re: Science feather strength debate
> To: dinosaur@usc.edu
> Date: Friday, November 5, 2010, 7:24 PM
> On 11/4/2010 10:09 AM, Don Ohmes
> wrote:
> 
> 
> > Also -- if some flightless birds retain asymmetrical
> vanes, while others
> > don't, and those birds that do not have them are
> "further" from their
> > earlier volant state --
> 
> 
> Some asymmetry is retained in those flightless birds that
> use their
> wings in some kind of aerial behavior. But yes, the
> distance from the
> volant state also has an impact on feather morphology,
> especially
> where ratites are concerned.
> 
> 
> The reference:
> 
> McGowan. C. (1989) Feather structure in flightless birds
> and its
> bearing on the question of the origin of feathers.Â
> Journal of Zoology
> 218: 537â547.
> 
> 
> > then that would tend to support the notion that
> > asymmetrical vanes evolve under flight conditions,
> would it not?
> 
> 
> I would tweak your sentence to read "asymmetrical vanes
> originally
> evolved under conditions associated with aerial
> locomotion."
> 
> This is from Garner et al. (1999; Proc. R. Soc. Lond. B
> 266, 1259-1266):
> 
> "For a flat, rectangular
> lifting surface, the centre of pressure lies approximately
> one-quarter of the chord behind the leading edge. If this
> point lies ahead of the rotational axis of the feather
> (i.e.
> the rachis), as in a symmetrical feather, then the feather
> will be unstable... A symmetrical feather can therefore
> only operate as a useful lifting surface if both rachis
> and
> socket are able to resist any torsional forces generated,
> or if the rachis is orientated parallel to the airflow
> (like the
> inner secondaries or central rectrices of modern birds,
> which are invariably more symmetrical than the primaries
> or
> outer rectrices, respectively). Thus, both gliding birds
> and
> powered fliers, possess asymmetrical primaries and only
> flightless birds lose this asymmetry (Speakman &
> Thomson 1994, 1995). Since arboreal and cursorial
> theories both consider flight
olution of the wing, both require that
> any feathers orientated across the airflow be asymme-
> trical as soon as they appear."
> 
> 
> 
> Cheers
> 
> Tim
>