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Ronald Orenstein wrote:

>>Before I consult Baumel et al.... time to ask the ornithologists - do 
>>the rib's uncinate processes have a known function?
>>"Now nothing can stop us in the raptorisation of Earth!"
>Without looking anything up, I have always heard that they provided
>stability to the rib cage, particularly during flight - but of course this
>doesn't explain their presence in non-flyers.  Still, some sort of bracing
>function seems at least plausible.

I'd thought that the uncinate processes were attachment sites for the
intercostal muscles.  Contraction of the intercostals exerts force on the
dorsal ribs caudally, which, in turn, acts on the ventral ribs (there's a
better term for this) which depresses the sternum.  In short, uncinate
processes are a marker, though not an infallible marker, for an avian
air-sac respiratory system.  Please let me know if I'm mistaken in this.

  --Toby White