I've got myself into a bit of a pickle, Dave. I normally run four bodies.
#1 - primary birding camera, attached to the best birding lens
#2 - dual-purpose wildlife and landscapes, attached to the 100-400 (I may have a 70-300L arriving by way of a swap for my old mark 1 100-400).
#3 - primary landscape camera, usually on the 24-105
#4 - dual purpose, wide angle and emergency spare (10-22 and/or 10-17 fish)
Up until recently, these were:
#1 - 7D
#2 - 1D IV
#3 - 5D II
#4 - 50D
This is the traveling rig, on arrival somewhere bird-rich I'd generally swap #1 and #2 to put the 1D IV on the 500/4 and then walk around with that plus one or both of #3 and #4
Then the 7D crapped out so I replaced it with a 7D II, and ordered various not-quite-so-ancient lenses and accessories, followed by the 5D IV.
The 5D IV is dual purpose. Mainly it is for landscapes with 24-105 or 100-400 (because I hate the brain dead 5D II AF system), but also for use with the 500/4 when birding in low light. (Rainforest especially.) That left me with:
#1 - 7D II (500)
#2 - 1D IV (100-400)
#3 - 5D IV (24-105)
#4 - 5D II with the 50D being sold. That requires a 16-35 or similar as the 10-22 is an EF-S. Unresolved was the question of what to do about the 10-17 fish, which fits on any camera but only produces its full zoom range on APS-C. I'm fond of that lens and use it quite a bit. There is no full frame equivalent. The Canon fisheye zoom is a bizarre and utterly stupid design which is no better, in reality, than a fisheye prime as you have near-zero zoom range without crippling vignetting. (I do not count a circular fisheye function as something any sane anthropoid over the age of 9 would ever use. Even Tea hates them.)
Then things got complicated. I had sent the 7D (mark 1) off for repair, more in hope than expectation, and it unexpectedly came back fixed, (Albeit with a $600 bill.) Now I have five bodies and I only need four.
The sensible thing to do would be sell the 5D II as it's such an annoying thing to use, and redeploy the 7D as #4 with the fish and the 10-22. But I had already sold the 10-22! Besides, for all of the 5D II's horrible AF function, it does produce beautiful low-noise images. On the other hand, the 7D is a joy to use. It's like having a brand new camera as although I've owned it for years it has always been used for birding with long lenses, practically never as a general walkaround.
I really don't need five cameras! If only I could have the 7D AF and handling with the 5D II sensor! I could sell both of them - seeing the back of the 5D II with relish and the 7D with tears - and put the money towards a 5D III in role #4, but that seems extravagant given that #4 is my least-used body. Besides, to use the fish I'd have to borrow the 7D II (which would be my only APS-C body) and I dislike taking bodies off the long lenses as I have learned the hard way that you never, ever leave yourself without suitable bodies on the birding lenses as the moment that you do some mega-rarity turns up and sits there in perfect light laughing at you. Believe me, this happens far more often than you'd think. Yes, I could work around that - both Mark IVs are very competent bird cameras - but it's a little messy, and it still leaves me spending even more of Tea's grocery money on a 5D III.
(The APS-H 1D IV is quite good with the fish too, by the way. You only lose the first two or three mm from the zoom range to vignetting, so it's effectively even wider than 10mm on APS-C. But all things considered it's still best to use APS-C for the fish where possible.)