Originally Posted by
neuroanatomist
The actual size of the AF point is larger than the box you see in the viewfinder (or in DPP). So, in your example it was integrating the bird and some of the tree branches in the focus calculation. My 7D offers 'Spot AF' which effectively uses a smaller portion of the AF sensor for more accurate focus. I have found that to be a big help when trying to focus on a small bird surrounded by a lot of branches (or in this case, a distant large bird).
In the shot above, if your goal was to capture the bird, you'd have benefitted from a longer lens, yes.