This has been an interesting thread to follow, as a wildlife (and bird) photographer and a Canon shooter. Whilst I don't own any Nikon gear I frequently shoot the newest Nikon cameras and lenses when I swop out gear with some of the folk who travel with us on photographic safaris. I like to shoot with the Nikon gear for many reasons. It is very important for me to have a good idea of the Nikon's setup and controls as I often have to assist photographers using Nikon equipment that are traveling with us. I also like to know the Nikon's capabilities, to the limit of my own abilities, and have an idea of the respective strengths and weaknesses for when somebody may ask my opinion on choosing gear for wildlife photography. I also would like to know which cameras are the best tools for the job, for my own shooting. In the course of leading photo safaris, I get to see the images from the Nikon and Canon systems displayed side by side all the time when we are processing, evaluating, critiquing our images with our groups of photographers.
From what I can tell, I would agree with some posters above that the Nikon D5 does a better job of holding onto a fast-moving subject like a bird in the air, whether using one AF point or the group of 5, or D21, than any Canon body, if one tries to track that subject when it moves against a dark background or a strongly structured background. I have seen that result with the Nikon 200-400 f4 VR lens as well as the Nikon 400 f2.8 VR v2 lens and also with the Nikon 200-500f5.6 VR lens and the Nikon 300 f4 VR PF lens.
On the other hand, the Canon 1DX and 1DX Mk 2 as well as the 5Dmk4, when used with the Canon EF 100-400L IS ii, the EF 400 f4 DO IS ii and the EF 500L f4 IS ii will all lock onto strongly backlit subjects (that my older Canon lens and body combinations would fail on) that are moving quite quickly, when the Nikon D5 fails to do so, mounted onto the Nikon 200-400 f4 VR and the Nikon 200-500 f5.6 VR. I have not had a chance yet to see how the newest Nikon fixed lenses fare in this regard as it may be the lenses and not the camera AF that are the trouble, but the end result is the same for the user.
For sure there are some differences in the autofocus capabilities of the two camera systems and their highest performing bodies for AF (1DX mk2, 1DX, 5Dmk4, 5Dmk3, 7Dmk2) and the D5, D4s, D850, D500 but I feel that they are fairly close to one another and the differences are not great in my opinion.
Lenses play a big role too in photographing fast action and up until now, Canon's EF 100-400L IS ii and the EF 200-400L f4 1.4x extender focus faster and deliver sharper overall image quality than their Nikon counterparts - the Nikon 200-400 f4 VR and the Nikon 80-400 VR, as well as the Nikon 200-500 VR. The new high-end Nikon zooms may change this a bit - the 180-400 f4 1.4 converter lens that is upcoming, and the newest Nikon 70-200 f2.8 - which is too short for most of the wildlife that we photograph.
Whilst it is fun to compare the high end of the two systems for action photography, I feel like they are closer to one another in autofocus performance than they have been in the past

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All our serious action shooting is usually done using either a Single manually selected AF point, or AF Expansion (One plus 4) or One plus 8 AF points, not AF Zone or the automatic AF settings. My wife Helena and I get to photograph a lot of wildlife in many different situations. We are mostly happy with the results we get from our gear but our own technique and application of skills is not always perfect, that we know for sure. We also don't expect that autofocus will manage to focus every super fast action shot that we attempt though we look forward to continuing improvements in AF.