What ISO levels do you usually shoot at for birding? If the lighting is bad, but you also have to juggle shutter speeds to stop motion, I would think you are at higher ISOs correct? If that is the case, the 6D2 will work just fine for you, its higher ISO levels are very close to the 5D4 in that regard. I wouldn't dismiss the 6D2 just yet if you find yourself at ISO 640 or higher. It also seems that the 6D2 has a reduced impact AA filter, based on some of the details that I have seen in various files at higher ISO, meaning NR can be more aggressive while maintaining details, and thus more usable than even before.
To give you an idea, I rarely go back to my 5D4 raw files. I shoot both raw and JPG, but with the proper in-camera settings, and the in-camera NR, the JPG files are actually very good to work with. I haven't been able to do that with prior bodies, other than the 7D2.
This is ISO 8000 processed JPG on the 7D2, the 6D2 will better this result.


My default ISO setting for birds is 800. It's only in Summertime that I can go lower to ISO 400. I also shoot a lot at ISO 1600. Depends on the quality of light and the location on how much noise that will give me but in fairly good conditions ISO 1600 will be very clean. I don't shoot at ISO 3200 unless it's an emergency.
I did notice how the 6DII files are pretty similar to those of the 5D4 at higher ISOs. What concerns me is the wiggle room. In winter I mainly walk the park and it gets dark there and I may have no choice but to underexpose. What I was wondering is this. The 6DII scores for DR are bad at base ISO but are actually better than quite a number of cameras at higher ISO speeds. Now I have only seen people pulling shadows and reducing highlights on files at base ISO and the 6DII does not do so well there, to say the least. However, as the 6DII is relatively better at higher ISO speeds in terms of DR, I was wondering how pulling shadows on files of ISO 800 or ISO 1600 would do. Or does the bad DR at base ISO also influence wiggle room in files with higher ISO speeds?
And thanks, Cary, for responding as I really do need help here. 



