birds vs. pterosaurs

[longrich@alumni.princeton.edu]

On Monday, July 22, 2002, at 10:03 PM, James R. Cunningham wrote:

> I don't see the contradiction.  I think there may well be an inate 
> structural
> limit for the size/weight ratio of feathers that limits how long they 
> can be in
> a flying bird.  This would act to limit the bird's wingspan.  That has 
> nothing
> to do with the seperate issue of comptetion (which I don't feel 
> strongly about
> anyway).  Even for a smaller bird the size of an eagle, the feathers are
> reported to weigh about twice as much as the skeleton.  But note that 
> in all
> fairness, this is an area that I'm not terribly interested in and 
> haven't given
> much thought.

	What I've read is that from a structural standpoint, in bending, a 
single spar is the strongest support , for a given amount of weight. 
Pterosaurs of course have basically just one spar supporting the whole 
wing, the forelimb-fourth digit. Birds on the other hand have multible 
tubular spars in the form of the rachi of the primaries and secondaries. 
So in theory this might be less efficient than having just one big one. 
Of course it has other advantages, like you can destroy a primary and 
still fly, and it probably permits a much greater variety of wing shapes 
(presumably more rectangular or elliptical wings for example as opposed 
to more triangular), multiple wingtips like in vultures, etc. There 
might be some other structural considerations regarding the 
feathers-v-skin wings thing. Bats would seem to be more structurally 
efficient than birds at first (relatively few spars) but they lack the 
ability to hollow out the wing supports via pneumatization so this means 
that their bones get much heavier as they get larger in diameter. Of 
course there could be other factors too.
	There are a couple instances I can think of where single-spar 
construction has taken over in human technology. Older fishing boats 
used to often have a tripod mast arrangement, with three spars 
supporting the mast, which had long since ceased being used to mount 
sail but which is still used to anchor stuff like a boom, mount sodium 
lights, radar, etc. The more modern arrangement I've seen in boats built 
since the mid eighties is to have a single large rectangular-tubular 
mast supporting all of this, presumably this is more structurally 
efficient. Cannondale bicycles also manufactures a rather 
bizarre-looking, asymmetrical single-spar fork (so I don't know if you 
can legitimately call it a fork). They claim it achieves similar or 
better strength for less material.