Thanks for the replies.Well, I'm no expert, but I've been using a 5d m3 for the past 6 weeks or so, trying my hand at BIF.
I always use the center focus point when shooting BIF so would I really need 61 AF points on the 5D MK3?
Does the MK3 need to lock on with the center point first before it will track the subject? Also, if the AF point drifts off the subject and is picked up by another point will it lock on to the closest part of the bird like the outer wing causing the head and body to be out of focus.
1) In general, depending on what settings you have and how you tweak them (and there's a lot to play with in the 5D), the 61 points are handy for BIF.
2) Depending on what objects are present around the bird (foreground / background), I switch from 61 points to 1, 5 or 9 points in the center, which is easy to do in the 5D (can't speak for the 6D).
3) It makes sense for the camera to focus on the nearest object, be that the body or the wing. It doesn't know what you consider important. But, in most cases (and I've only had a few), it's more luck that the camera happens to be in focus on the body (or the bit I want in focus), since the time frame I'm dealing with the bird is around 1-2 seconds
Just my 2 cents worth