I suggest going ahead and buying the 7D II from a retailer with a great return policy (B&H, Adorama, etc.), but don't sell the 5D III untill you try out the 7D II. If it's all that Canon says it is, it really should be a crop-frame "1DX" (in terms of fps, processing power, and AF prowess) for wildlife imagery. And if you love it, then throw the 5D III up on the POTN sell forum and find it a new owner. Or, if you don't like the 7D II, return for a full refund.
And, frankly, that is what the retailers want you to do...they've got those great return policies to make it easier for us to make these hard decisions.
Finally, a word about photography: Focus your passion on what you love (birds, wildlife), and don't worry about the side stuff - the occaisional performing arts shots, etc. Do the one thing and get great at it (really preaching to myself here!).
Best wishes on your decision, it's an exciting time.