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Re: feather asymmetry

 Alan Brush <brushes2@juno.com> wrote:

> Regarding the evolution of feather asymmetry see:
> Feo, TJ and RO Prum 2014 Theoretical Morphology and Development of Flight
> Feather Vane Asymmetry with Experimental Tests in Parrots. JEZ  DOI:
> 10.1002/jez.b22573.
> Discussion includes steps that must have occurred in the evolution of
> vane asymmetry.

It certainly highlights the developmental complexity required to
produce an asymmetric vane.  Very interesting.  Thanks for the ref.

The same study also mentions the aerodynamic utility of an asymmetric vane:

   "Whereas the particular direction of asymmetry in most
   organismal examples does not appear to serve a specific function
   (Palmer, 2009), the direction of vane width asymmetry in flight
   feathers does confer specific aerodynamic advantages (Norberg,
   '85). Both the direction and degree of vane asymmetry contribute
   to the control of individual feather twisting in airflow during flight
   (Norberg, '85). A relative reduction in leading vane width
   effectively shifts the central shaft, or rachis, of the feather closer
   toward the leading edge. Depending on the degree of asymmetry,
   the rachis is either positioned at the center of pressure, which
   minimizes feather twisting in airflow, or ahead of the center of
   pressure, which causes feathers to twist like venetian blinds;
   opening on an upstroke and allowing air to pass through the wing,
   and closing on the downstroke to create a continuous surface
   (Norberg, '85). This suggests that both the direction and degree of
   vane width asymmetry in flight feathers are under strong natural
   selection to maintain a functional aerodynamic phenotype."

>From this I infer that an asymmetrical feather is useful for
powered/flapping flight, but doesn't necessarily mean powered/flapping
flight.  Asymmetry reduces torsion of the feather in response to
airflow.  So asymmetry would be potentially useful for aerodynamic
behaviors that might not entail a wingbeat: such as gliding or
controlled descents, or simply maintaining balance or maneuvrability
on the ground.  Also, the aerodynamic remiges and rectrices were first
developed distally on the limbs and tail, which is consistent with a
function in control more so than lift, IMHO.