An update...
I have been testing a Sony a1 + FE 400mm f.2.8; even if I need a lot more practice, I believe to "have seen enough” to share these
preliminary thoughts (which I believe are in line with the previous comments on the thread from pretty much everybody):
1/ The “BIF problem” described at the beginning of this thread is complex, you have to accept a low keeper ratio
2/ The a1 is better at this problem than the D850 because of (in a nutshell): (i) much higher frames per second, (ii) faster focus acquisition and (iii) lower weight (relative to the D850 with an equivalent lens). Lower weight in particular makes it a lot nicer to shoot
3/ The a1 is by no means infallible; the keeper ratio is somewhat higher but not stellar (for a given level of photographer!)
4/ The Zone AF tracking seems the best for this problem but still loses the bird a good percentage of the time and hardly ever gets the bird on the ground. Spot S AF gets the bird on the ground and loses it in the air
5/ Eye detection (for this problem) did not impress me… unless you get really close on a static bird it does not seem to get the eye that often at all. It did work very well on my pointing dog (see below)
6/ I might need to get used to it but did not particularly enjoy the EVF… other than the no blackout between shots
7/ The a1 with the FE 600mm f.2.8 is the better option for this problem. It can probably be shot handheld… little chance of doing that comfortably with a D850 and a Nikon 600mm
8/ You pay top dollar for those additional keepers you get (many $$$ for few additional keepers)… it is debatable whether it is rationally worth it, particularly when there is (and it will only increase) a lot of good quality DSLR lenses coming to the second-hand market…
9/ Shooting this problem with the a1 was tremendous fun and perhaps a touch less frustrating than with the D850
Some pictures (mostly BIF):
1/
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2/
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3/
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4/
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5/
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Thank you to all once more!