Best mirror less camera to pair with a Canon RF 100-500mm F4.5-7-7Lis

Am mostly into bird photography but can no longer carry the weight of a DSLR and my Tamzilla 100-600. Am just too old to carry around all of my heavy glass and cameras.What's the best mirrorless body I can use (and tote around) with the Canon RF 100-500? Would prefer full frame.

Thanks,
Betty

Please tell me what is the best mirrorless camera to use for photographing birds?

I don't think there's any one answer. I'd look through the bird pics and see what people are using for the subjects that you have.

If you're shooting small birds then the R7 (crop) has more pixels on target than nearly anything else. Plus it's small and light. If you are shooting large enough birds that you can fill the FF sensor, or are willing to TC, then let your budget guide you through the FF models. None of them are that big until you get to the $5600 R3.

I think there is an obvious answer. Canon R5. You said 'prefer full frame'.

Some people have chosen the R3 over the R5. They sacrifice resolution (45mp R5 vs 24mp R3) for the various advantages of the R3.
It is much more expensive and it's heavier.
R3 1,015 g (35.8 oz)
R5 738 g (26.03 oz)

However, as you want to reduce weight, I think that rules out the R3.

R6 is 20mp and R6 MkII is 24mp. I think everyone would agree 45 mp is better for birds than 20 or 24.

R7 is crop. If your full frame preference was negotiable, the R7 should be considered.
If budget is not an issue, R5 and R7 would make a great pair.

Thanks for the info. I'm leaning toward the R5 or R7.