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Re: Science feather strength debate

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.  Journal of Zoology
218: 537–547.

> 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 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."