The nice thing about living at this time in photography's development: there are lots of very good options for wildlife/birding lenses at prices many of us can afford.
I had a 400 f/5.6. I'm sure it was just me as I've seen brilliant photographs from this lens, but I couldn't get anything decent, no matter what I tried. I don't think it was the lens.
Anyway, I was able to buy the 100-400ii and my bird photographs improved immediately. I've had it for a little over a year and have used it through two Spring breeding seasons. These days I have it on a 1D IV and couldn't be happier.
I've also seen very good photographs from all the lenses you mentioned. A question: how close can you get to the wildlife/birds in your location? That could impact your decision. If you are taking photographs of smaller birds, you'll have two issues: getting them to fill the frame and having a lens that focuses quickly as they don't sit still long. An example: I mostly shoot the large wading birds here in Florida. They aren't that speedy in the air and they stand for long periods of time when they hunt. I can get close to them (for the most part, they aren't afraid of humans).... close enough that I seldom need 600mm. On the other hand, the little song birds in our backyard sit on a branch for a second or two, then to the feeder for a second or two.. and are gone in a flash. A fast focusing lens that goes out to 600mm would work better for them.
The good news? Any of the lenses on your short list will do the job at hand. When I got my birding lens, the Sigma offerings were not yet available, so I wasn't able to research them. I have read a lot about them here on POTN and people who own them seem quite pleased with their performance. One thing about which I've heard people complain? Weight. I'd look at the specs for all the lenses you mentioned..... make sure you take the weights into consideration. The 400 f/5.6 is the lightest, obviously.... but it doesn't zoom. For me, I needed a zoom because there are many times when 400mm is just too close.
Anyway, those are some of my thoughts on your question. I'm sure whatever you decide, you'll enjoy the 7D2/lens of your choice combination.
Kim (the male variety) Canon 1DX2 | 1D IV | 16-35 f/4 IS | 24-105 f/4 IS | 100L IS macro | 70-200mm f/2.8L IS II | 100-400Lii | 50 f/1.8 STM | Canon 1.4X III
RRS tripod and monopod | 580EXII | Cinch 1 & Loop 3 Special Edition | Editing Encouraged