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birds vs. pterosaurs




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.