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On Matt's 15th May piece on ornithoptering, I'm sure Archae. could flap-fly
well - but. . .

< Previous work done on starling
furculae by Jenkins et.al. show that the furcuale of modern birds acts
as a spring-spacer, a necessary part in the wingbeat and lung
ventilation.  There are exceptions to the rule such as falconiforms and
parrots where the furcula is to robust for being a spring and too flimsy
to be a spring respectively.  The main way that the furcula becomes a
spring in modern birds is its rounded cross-section.  The rounded
cross-section gives it more flexibility.  However,  the furcula in
Archaeopteryx and the enantiornithines has a thin,  flat cross-section
because the furcula is posteriorly grooved . . .>

Sorry to sound a bit contradictory, but the falconiform (kestrel -
presumably one) furcula I have in my hand at this very moment has a thin
flat cross-section over most of its length, except at the midline (the
stress concentration area) where the dimension ratio of its x-sect
approaches 1.5:1 .  Also, I would have thought that a rounded cross section
would give less flexibility, not the other way round.

(No doubt you will all know this but. . . it's shaped just like a capital-U
from the front (also a bit like a human lower jaw bone pair) and seems to me
to be closer to the Archae. boomerang than chickens/turkeys.)

It's the most spring-like bone in the body but strangely is probably too
springy to hold more than a small fraction of the wing kinetic energy,
unless it forms some sort of "composite bow" structure with additional
ligaments etc.