Quote Originally Posted by Fast Glass View Post

If shot on a R5, it can be used in 1.6 crop mode to reach an even more staggering effective focal length.

I got a chuckle out of that one.

Thinking on the opposite end though and using the R5, the 800mm has a minimum focal distance of 8.53 feet and a magnification of .34x.
Compare that to the 600mm has a minimum focal distance of 13.78 feet and a magnification of .15x. The 600mm would be 63% farther away.
If you are photographing small birds at the minimum distance that you may need to crop you will have far more pixels on the subject. The .12" of DOF might be a little tight. At .405' x .27' FOV many birds may not fit the framing.