Avian respiration

["Michael" <tons@ccs.netside.com>]

Someone recently wrote that there was gas exchange in the air sacs.  
I don't think that they have the necessary circulation for gas 
exchange, but if someone has references I'd like to correct myself.

I'm very interested in anyone's ideas on how avian respiratory 
physiology and anatomy started, because I can't see a progenitor in 
the reptilian line.  Your thoughts??

Michael Teuton
803-732-2327 Phone
803-749-6191 Fax