Why did the vertebrae of birds fuse together?

["Henri Rönkkö" <overmind@iobox.fi>]

 Dear all,

Why did the avian thoracic and lumbar spinal column fuse into a solid clump? 
Dinosaurs and primitive birds have unfused vertebrae. If you have looked into 
this matter, it would be nice if you shared your views with me. I'll now 
present some of my own.

I've read that this fusion made the back stronger. This probably means that 
some expectionally strong forces are exerted on the back as the bird flies. But 
where do these forces come from?

A thought of mine and quite possibly someone else's too is that the stiffened 
back is advantageous to the bird because a flying animal with a flexible back 
would deform all the time, and thus its aerodynamic properties would be spoilt. 
Indeed, humans have built no physically, literally flexible flying machines. 
Flexing the back might also change the moment arm of the guts about the 
glenoid. This would happen all the time, thus making the bird unstable.

Thanks in advance,
Henri Rönkkö
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