Re: Science feather strength debate

[Jason Brougham <jaseb@amnh.org>]

Feather asymmetry is a complicated topic.

I've talked with people who have suggested that asymmetrical feathers may have 
originated for some non - aerodynamic purpose, like tighter wing folding, but I 
would tend to think they evolved in a gliding animal with symmetrical feathers.

Greg Erickson pointed out that asymmetrical veins are not necessary for lift - 
a frisbee gets lift. He said that the asymmetry actually functions to allow the 
feather shafts to twist during a  flapping stroke - they open and allow air 
between them on the upstroke (reducing air resistance), then slam shut like 
louvre blinds on the downstroke, increasing resistance. Then I  wondered aloud 
if Microraptor couldn't have flapped its legs too! It seems ridiculous, but 
maybe the little bastards pumped their legs like bicyclists in mid-air! I don't 
seriously think so, but Microraptor is such a  puzzling animal. I don't know if 
I'll ever feel certain about anything it did.

I should mention, though, that a priori and comparative conclusions are useful 
and fascinating, but we always have to be ready to be persuaded by new evidence.

As examples, some have argued that a bird can't engage in powered flight 
without a supracoracoideus, but then Sokoloff et al. (2001) found that 
starlings that had had their supracoracoids surgically removed could take off 
from the ground just fine. 

When I was working on the NOVA episode The Four Winged Dinosaur I was sure that 
the asymmetrical feathers on Microraptor's feet had to face toe - first into 
the airstream. Logic dictated that asymmetrical feathers wouldn't provide lift 
if you flew them backwards. But Dr. Xu wanted to try them backwards, and we did 
it, and we got great lift from backwards asymmetrical feathers in the wind 
tunnel! 

That's why, when we have discussions about extinct species based largely on 
logic and comparative data, I always start to yearn for some experimental data 
to test things. A bad experiment can mislead you, of course, but a good one can 
be  a crucial reality check.

On Nov 4, 2010, at 10:44 AM, Don Ohmes wrote:

> 
> > On 11/4/2010 9:14 AM, Mike Habib wrote:
> >> Not to mention that asymmetrical feathers are not actually required
> >> for flight, from a first principles standpoint, despite the now
> >> popular notion that there must be a 1:1 correspondence.
> 
> >>> On Nov 4, 2010, at 7:19 AM, David Marjanovic<david.marjanovic@gmx.at>
> >>> wrote:
> 
> >>>> However, some flightless birds retain asymmetrical vanes (e.g.,
> >>>> flightless grebes)
> >>>
> >>> Oho. Reference, please?
> 
> On 11/4/2010 10:09 AM, Don Ohmes wrote:
> 
> > The fact that some flightless birds *retain* asymmetrical vanes should
> > not negatively affect the status of asymmetrical vanes as indicators of
> > volancy.
> 
> 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 -- then that would tend to support the notion that asymmetrical 
> vanes evolve under flight conditions, would it not?

Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544