If you only go birding occasionally, I'd stick with the 50-500 for that - in reasonable light it performs well, especially if stopped down to f8. (And the 1DsII will give relatively smooth results at high ISO).
I think Joe has given some sound advice if theatre/stage useage is your mainstay. (And of course, with a 300 f2.8, you can add a 2*TC and have a 600 AF combo for birding)
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