Hi Mike,

Congrats on the R5 & RF800. I have been shooting birds with that combo since December 2020. It is a super combo to do just that.

Here are a few examples of BIF's with that combo:
https://flic.kr/p/2m4JgQx
https://flic.kr/p/2kLiNs1
https://flic.kr/p/2kBomo6
https://flic.kr/p/2khXB5J

The settings I use are:
I use dual back button focus. For BIF's, the AF-ON button is for Large Zone AF: Vertical (page 139 of the R5 manual). The astericks button next to it I use for Face Tracking / Eye Detect (page 137 of the R5 manual). I switch between them as the need arises. I have used each of those buttons for BIF photos and I do not see a difference.

But, there's more about quick selecting the AF Method. I have programmed the Quick Control Dial 1, on the back of the camera, to scroll thru the AF Methods so I can select which mode works best for a given situation. Then I use the back button focus (AF-ON) with the selected AF Method. This works excellent if I'm trying to point focus on a subject in a crowded environment (a single bird in a tree or bush).

Now, I have settled on these AF parameters. I get the best keeper rate for perched, swimming or flying birds using:
AF Menu 3, Servo AF Case 2. Tracking sensitivity -2, Accel / Decel 0. I did use Case 1 for thousands of photos and wasn't impressed so I switched to Case 2 and find that works much better.

Shutter speed is 1/4000 when I turn ON the camera (I set ALL of this up on Custom2 (C2) Mode). I use the lens ring to adjust shutter speed. I find best success when shooting at least 1/3200. I thought 1/2000 was OK but I get way more keepers at 1/4000.

I have now used eye detect for BIF photos as well as Large Zone AF: Vertical and both work well.

Stay a little in front of the bird when you view it thru the view finder. I don't photograph small birds in flight. It's too hard for me. Never had any success at it. I much prefer larger birds. Raptors like hawks, falcons and eagles are my faves. They are highly intelligent and will often circle around you to check you out. That's when you can get some awesome pics. Position yourself so that the sun will illuminate their face.

If you have anymore questions, just let me know. Glad to help.

I also use the 100-400 II with the R5 with an adapter, for wildlife picture and get superb results:
https://flic.kr/p/2kXD98i
https://flic.kr/p/2kXDLFR

Feel free to PM me with any questions. Glad to help.

Hi Mikey, I usually shoot in shutter priority mode. Lately I have been also experimenting with Fv mode too.

For shutter, I use Elec. 1st -curtain. If I need high speed, I use Electronic shutter.

I use an UV[0] lens protective filter on all of my lenses.

Also, I encourage you to experiment with the SERVO AF modes 1, 2 and 3. See which modes give you better keeper rates. For instance try Case 2 with -2 tracking sensitivity and 0 Accel./decel tracking. I find that Case works real well too.

For sure I'll have to experiment to see what works best for me, I confess that I don't know much about Elec. 1st curtain shooting and only a little about using the electronic shutter shooting, I've always shot using the mechanical shutter with my DSLR in the past. Like you, I have a protection filter on all my lenses but on some of the vids I've watched say they don't use them because they have an impact on image quality, I've never tested the IQ difference.
Also like you, I'm set up with Case 2 with track/sens at -2 but my accel/decel is at +1, I'll see how that goes. I've set up my Servo AF with 1,3 and also the wide horizontal zone AF....I'll see how that goes too. Thanks again Art!