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Re: Science feather strength debate



Indeed, feather asymmetry is complicated, to an extent. Greg is mostly correct 
in his description, but not quite precise. The asymmetry dies affect twist, and 
feather twisting in the outboard wing is indeed important (though they twist on 
both strokes, and 'air resistance' isn't quite the right term, but I know what 
he's getting at).  The catch is that symmetrical feather can provide the same 
kinematics, but it requires more material.

Cheers,

--Mike H.

Sent from my iPhone

On Nov 4, 2010, at 11:32 AM, Jason Brougham <jaseb@amnh.org> wrote:

> 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
> 
>