Many of us, in contrast to moocowe, have found that we have to take several shots when doing bird photography with the 7D to get tack sharp focus whereas the 5D III is spot on every time.
Sorry, I should have stated I don't do any bird photography. Closest thing I've shot was an airshow, where I thought the AF performed really well. That was just after I'd stepped up from a 450D, and long before the 5D3 existed, however!
No doubt the 5D3 AF is superior, but the 7D AF should also be quite an improvement for someone coming from an 1100D.