"Lastly, as for the Tamron, I thought someone said it is pretty big and would need a suitcase to carry around. Unfortunately, that would defeat my purpose of trying to reduce weight."
Don't believe everything you read! Except form me of course! 
In the unzoomed condition it isn't a great deal longer than the 70-200mm f2.8 with the tel-con attached. Is it bigger, yeah, it is. It is 600mm of FL. Like I said before weight and birding go together. There is just so much that can be done on the weight side before you compromise on IQ.
@ebiggs1 wrote:"...the R as the prototype for Canon's serious mirrorless line. It's not something you'd buy now."
There is no way on God's green Earth I would trade a 5Ds for a R or much less a Sony!
I agree. The buffering in the top-of-the-line Sony bodies is atrocious. A burst of 20 shots, which is easy to do at 20 fps. It can take MINUTES to clear out. Meanwhile, you are not able to take more shots until the buffer clears. The R5 sounds promising!
----------------------------
"Anyway, what is your thought of 400mm f5.6 USM L lens? The reviews I have seen seem very positive. It's a prime lens therefore it is sharper than zoom lens. Also, it is not as heavy as the 70-200 f2.8 II or the 100-400 mark II."
All reports say that it is an excellent lens for wildlife. I would prefer to use it on an APS-C body like the 7D2, though, so that I can get the "free" extra focal length from the cropping. Shooting with a super telephoto prime is a very different experience from using more normal lenses. There is a definite learning curve associated with using a super tele.
You are viewing the world through a straw. Some people can acclimate to it, while some can never get the hang of finding their subjects while looking through a straw. As good as the lens is, I would still recommend a super telephoto zoom over the 400mm prime for that lone reason.
For birding, the weight savings of mirrorless don't mean much since you will have a heavy chunk of glass attached to the camera 🙂
And don't sell the 5DS R short, it isn't my first choice for low light, fast action sports (I have 1DX through 1DX III bodies which excel under those conditions) but the 5DS R does quite well under a wide range of conditions. I took it to one HS basketball game as a third body for bench/crowd shots with a 24-70 F2.8 lens but took a few action shots with it just to test and below is one. It doesn't compare well to what my 1DX series bodies did with 300 F2.8 and 70-200 F2.8 lens mounted for that game but it was definitely handicapped by its less speedy focusing 24-70 lens and was better for sports than I expected.
The shot below was at ISO 2000 but I bumped it up nearly a full stop in post and it is highly cropped from a larger capture.
I shot a lot of soccer with a Canon 400 F5.6, often with a 1.4X in place and it is a very nice lens and one of the standout value for price lens in the Canon line and if it is long enough for your needs it does a great job. I switched to a Canon 400 F2.8 which is even sharper and takes extenders with less loss of sharpness and contrast but that comes at a huge dollar and weight increase.
I had a chance to pick up a 5DS R at a compelling price and I found that I use it far more than I expected. The only change I made was adding the Canon battery grip because I really missed the portrait orientation duplicate controls of the 1 series bodies.
Rodger
