Simon
if the reach of the 100-400 is just a little short for you then maybe you need a baghide...just a thought.
I have a lot of experience with the Sigma 300 prime. At 600mm you will not have the quality of a bare Sigma prime and sometimes not the quality of a Canon equivelant at the 600 range when viewed 100% but I can assure you that you can still do large quality prints. I did one recently and cropped about 50% and still got an A4 print that would be of publishable quality no sweat, and I am sure could have been printed larger. If you pixel peep you will find fault with any lens and that includes the new Canon 400f2.8 Mk2!
You don't state your budget but it would seem to me that if you cannot get a Canon second hand then you may have little option but to go for the Sigma 300 f2.8/ Sigma 120-300 OS if possible or consider a 50-500. Aside from IQ there is focus to consider and the Sigma prime does not have focus limiters. Now this can be a bummer but with anticipation and good practice you can minimise the effect by pre-focus which is a sinch.
On other forums I have found comments on cameras and lenses to be a complete and utter joke...I have never read so much tosh and rubbish in all my life regarding focus acquisition of lenses and cameras...shots of lumbering cranes and eagles blah blah blah to justify the latest camera or lens, it makes my blood boil! If you need state of the art equipment for these birds then I would give up. If a sad old amateur like me can nail peregrines diving at me with an old Sigma stuck on a 20D body then anything is possible...so do not feel disheartened by the fact that you may have equipment that is 'inferior' to the 'gold standard equipment' users. The Sigma 300 f2.8 will in my view be an upgrade for you but it is an entirely different beast to what you have been used to. You may need a good tripod with a good head and give yourself time with the learning curve. If you can afford the Sigma 120-300 OS then that is the one to aim for but if you cannot get it then don't ever think that you cannot get cracking shots with the alternatives because you can.
The attached is a woodpigeon taken recently , jpeg capture (no in camera sharpening) Canon 7D , Sigma 300 f2.8 ,420mm, ISO 500 , f5.6 no post processing or sharpening other than resizing and compressing for the web which has induced compression artifacts. It follows that the original will be sharper and sharper still when sharpened.
The Iris was taken with a 2X converter and lifted off my gallery so is processed.
Finally, if you do go for the set up, based on my experience, there is a quality upstage by using a Canon 2X MkIII converter, there is certainly less chromatic aberation than there is with the Sigma and Kenko converters that I have used.
