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Whatever. I don't recall meeting any bird photographer that uses the back button. To each his own.
Regardless, back button or shutter button, makes no difference to AI Servo AF.
I would say of the GOOD bird photographers I know at least 90% uses back button focusing, mind you most poor to average bird shooters do not use back button focusing I will admit.
With all due respect, Imagemaster --- Regardless whether the subject is birds, football players, or bridezillas, I do not know a SINGLE serious (pro or amateur) Canon-using photographer that DOES NOT use the back focus button.
OTOH, EVERY Canon shooter that purchased his or her first Rebel last week, uses the shutter button....... most of them switch when someone shows them the back button and explains the advantages.....
...Said advantages are too great to deny.... Having the equivalent functionality of AI Servo and One Shot at the same time and without having to change a setting PLUS the ability to focus and recompose while holding exposure for those that do not possess two thumbs on the same hand.... who would not want that....
Just my .02....