R5 benefits
Note: I have severe physical limitations on how I do my bird photography. These lead me to depend on rather extreme equipment and settings, so it may not matter to you at all, if you don't do things like I do. First, I use the longest photographic lenses possible, most often Canon 600/4 with 2x teleconverter; second, I'm a pixel peeper, 50 Mpix is not too much, especially since I always crop, often heavily; third, I often work in dismal, dreary, overcast and rainy conditions, f/11, 1/500 sec at ISO 1600 is normal; fourth, I always work from my car window, space is a severe limitation, and I ain't small.
Autofocus: This is a big difference from all the other Canon DSLRs I've used (5dii, 7dii, 5ds&R, 90d). The R5 uses it's full AF features with lenses of up to f/11 max aperture (thus, the new f/11 lenses), those DSLRs I have used lose AF versatility starting at f/5.6, are limited to the center point at f/8 and AF is gone at f/11. The R5 AF seems to work better under low light/low contrast conditions (think of dingy, dark birds under a tree in the rain). It seems as fast as PDAF. Some people on the web have commented that they are bothered by the jittery blue boxes; I find them to be helpful to see just where the AF system is selecting details to focus on; I suspect that the DSLR cameras' AF systems do something like this, too, but you can't see the points they are using jigging around, only the general area of the AF sensor and that may help explain why I get so few critically sharp images. It is my impression that I'm getting 2 or 3 times as many critically sharp photos with the R5 than I ever got with DSLRs, including instances where I've been attributing poor focus to "atmospheric distortion".
Reframing with AF: Having the AF system detect a subject's eye or head and then hold on to it while continuing to AF as I reframe the image before clicking the shutter, means that I can make the most of the full sensor area, rather than having to hold the eye or head on the center AF point (max aperture f/8) or other selected point (when I can use f/5.6 or below) and then have to crop out major areas of the frame for better composition because the bird moved around during the process. With the DSLRs this could only be done in OneShot mode, which means compromising critical focus with a constantly moving subject.
High ISO: ISO 1600, for me, is not high, but it is the limit I've used with previous cameras due to excessive noise that requires more extensive work in editing and print preparation at higher ISO settings. It looks like the R5 is just about as good as the DSLRs but extends up to ISO 6400, that's two stops of more useable ISO range, which is very welcome for me in Oregon's typical dreary winter weather. I do use a higher level of default noise reduction in LightRoom, but don't see any bad effects that I would expect from the DSLRs. I don't know how technically meaningful this is, but I like it!
Flip screen: Having a high resolution sensor and a flip screen is the reason I bought a 90d; to my mind, it was a tremendous oversight for Canon to have not included one on the 5ds bodies. I don't use it often, but when needed it is an image saver. I've had to use an angle finder on the DSLRs to accomplish the same thing, but it's not nearly as versatile.
