For the most part D850 BIF settings are the same with a D5/D6. The D5 does introduce some unusually shaped and handy Group AF Areas useful for things like panning with horizontally flying birds. The D6 takes that a step further and allows you to customize two Group AF Areas which can also be very handy for certain BIF situations. Here's a look at the customizable Group Areas supported by the D
That's taken from this Nikon page which has more discussion on D5/D6 AF settings: https://nps.nikonimaging.com/technical_solutions/d6_tips/af-area_mode/index.htm#auto-area_af
Other than the new Group Areas I used the same AF settings with my D850 and D5.
Thank you, I think I'll probably set it up for 5x5 for birds in flght and single point for perched birds and leave those locked in to toggle back and forth.