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Just curious, any other advantage of the 5D3 over the 1D4 in birding, aside from improved AF?
Well it's not a huge difference, overall, but if you aren't FL limited, the extra pixels of the 5D III allow for some great feather detail and you do notice the lower noise above ISO 1600. I'd say it's > 0.5 stop better above ISO 3200, probably closer to 1 stop at ISO 6400+.
It's nice on the 5D III, like the 7D, to be easily able to change your AF point from spot to normal to expanded etc with the MFn button. So much more customisable than the 1D IV. All you'll really miss is the rugged build and extra speed and better battery, although the 5D III still can get 1000+ shots and if you use the grip you'll double that.