It's true, I'm asking quite a lot of the technology and I suppose to be able to identify the bird on the treetop at all is pretty good. I was hoping that the focus would catch up to the owl when it was flying, but it just wouldn't reacquire it at all. Ironically this might be due to the settings I chose, which enhanced the lock-on tracking. Once the focus decided that I really wanted to take a picture of the background trees, it ignored the bird completely. So it's a setting that cuts both ways, I suppose. Maybe if I had set the focus mode to quickly jump to the nearest subject, it would have found it again.

With regards to the single-point focus, yes the standard point is rather large. The spot-AF point is about half the size, which should work better in the bush. However you probably give up some accuracy when you make the point smaller. One thing that I do like about the 1DXIII versus the R5 is that the focus points are very small. I don't even bother activating the spot-AF mode on that camera. The R5 single-point focus boxes are quite a bit larger. But then, you get coverage over nearly the full screen which is quite an advantage.