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I go out on a kayak with my camera, and usually the GBH are pretty skittish. Times I've gotten closer (not as close as yours) have been when one was being successful in it's fishing and reluctant to give up a good spot. I've found the medium sized birds such as a green heron or smaller egret to consistantly be more tolerant.
Obviously, your demeanor put him at ease, so he didn't consider you a threat and you were just too big to be considered a treat.
Nice shots. Thanks for sharing.