I have just bought (still not arrived in my hand) a 1Dx & a 5D3 camera & the new 400 f2.8. My personal passion is photographing birds in flight, but I am a rank amateur. I have read, very carefully, the Canon AF setting guide book but am still uncertain on one big question - what is the best area selection mode to set up on the cameras to ensure that the focus grabs the subject (bird in flight) & thereafter continues to track it accurately. Do I set up a single point AF or do I use the auto selection of 61points. My feelings are that the single point AF will allow focus capture of the bird, especially when there is a lot of background clutter, more easily but thereafter keeping the single focus point on the bird is a real challenge, especially with the heavy 400 2.8. The auto selection of 61 points that, from my understanding of the handbook, allows the AF to shift to any of the other 60 points if the original focus point that captured & locked the focus moves off the bird, seems like the perfect answer. However, the auto selection is fine when focusing on a bird in clear blue sky, it should easily achieve focus lock against the uncluttered background. My question is how will it pick up & focus lock & track a fast flying bird against a cluttered background. Opinions from the experts out there would be really appreciated.