Okay ... walking around in the Zoo with a camera backpack is aggressive and quite honestly, a pain in the rump. I analyzed my requirement during the last trip and began to pare down the lenses required (loosely used) for such a trip.
The sling bag is a much more manageable item to use. One aspect that may be a little bit of an issue is that forces me to hand-carry the monopod, if I want to make good use of the 200-500mm. I guess I could rest in on a fence rail, if I go without it. Tough call. It works handheld, well enough. It seems much more at home on the SONY DSLRs than the results I received on the Canon.
Okay, the sling bag currently contains the 17-50mm f/2.8, 90mm f/2.8, 135mm f/1.8, and then the 200-500mm has its own case, which I can tightly tether to the sling bag or tote via its own sholder strap. It is simply too big to place in the sling bag, itself (the price of long glass, I guess).
Obviously, using the 80-400 would be compromise the entire shoot ... as I am after some seriously sharp and wide-open shots and it is restrictive, indoors, to f/4.5-5.6 ... that, my friends, is flash country.
Anyone seeing the reasoning behind the 70-200m f/2.8?
Oh well ... third times a charm ... I hope. :D