There's more explanation of the 7D AF system here -
http://www.usa.canon.com/uploadedimages/FCK/Image/2010/QuickGuides/CDLC_EOS7D_AF_QuickGuide.pdf - including recommendadtions when to use certain modes and when not to.
My own personal experience of BIF photography with both the 7D and 1D3 is to use the smallest number of AF points possible to get the shot. I will often use a single point for BIF, now that my tracking skills are a little better than they once were, but if I'm struggling with that then I will usually go to expansion before zone. I have rarely used all point AF with either camera. I'm not convinced that the results are as good as with fewer points.
There are a couple of points to consider here. Firstly, the more points you activate the more data the camera has to process and the more decisions it has to make about which point is the right one at any given instant. The easier you make things for the camera the more reliable the results will be. Secondly, if you are photographing a large bird, perhaps a heron flying across your path, something like zone AF is more likely to pick up on a wing tip and focus on that rather than keeping things sharpest on the head/eye. Thirdly, if your subject blends in rather too well with the background and the background is contrasty and competing for attention then again using a single AF point of tight AF cluster will help ensure the AF is working on the right part of the scene, subject to photographer skills.
I find the same thing with my running dog photography. Results seem better when I use a single AF point to track the dog. That makes my task more challenging, but the camera can deliver better results.
The camera certainly has features to help you out, but I think there is no substitute for good skills at these difficult subjects. So to recap, use as few points as you can manage with. There are other techniques and settings to help out. For example, I usually set tracking sensitivity to slightly slow or slow. This gives me a moment of respite if I lose track of the subject in the crosshairs before the camera will wander off and find something else to focus on. Also, with back button AF I can simply cease AF momentarily if I lose track. This avoids the camera drifting off completely. Once I am back on target I can resume AF once more.
Lastly, when shooting action, and especially BIF, I find that a high shutter speed is usually key to success, and if you're going to pixel peep 7D images it's pretty much mandatory. For a hand held 400mm lens shooting at 1/2000 or faster is going to get you more keepers than shooting around the 1/1000 or 1/800 mark. It can be easy to confuse poor focus with camera shake or subject blur.
