With Sony about to release its 600/4 supertele prime, and already having a 400/2.8, we now have three separate systems ready for dedicated, high-end wildlife photography with full-frame bodies. But just because multiple options are available doesn't mean carrying multiple superteles in the field is all that feasible, whether due to weight when carrying them, airline luggage restrictions (particularly on small planes) or simply the expense of owning them. One must be selective in what they carry, and usually this means one big lens, as well as a secondary lens.
For flying birds, it's pretty obvious - as long as possible.
Dense jungles and places like the Galapagos Islands are another obvious exception - animals are close enough, or line of sight is limited enough, that shorter focal lengths are frequently in play. A 180-400/4 or 200-400/4 on full frame, with a 70-200 as close-range backup, would be good for these environments.
But what about for other, non-birds-in-flight wildlife in more open areas - large game on the African savannah, black bears and grizzlies in forests, penguins in South Georgia or polar bears in Svalbard?
The way I see it, there are several viable combinations:
1) 800/5.6 and a zoom (100-400/4.5-5.6, 200-500/5.6, 200-600/5-6.3 or 150-600/5-6.3) - long reach, but the prime is going to be too long for many closer or larger animals, leaving you heavily reliant on the secondary lens, and with limited options in low light.
2) 600/4 and 100-400/80-400/other similar zoom - a much more flexible big lens, with better low-light capability and more ability to use it for closer or larger animals. You're still going to have to resort to the backup lens in many cases, although not nearly as often as with the 800/5.6. With teleconverters, you can reach 840/5.6, or even 1200/8 in a pinch, although that may be really pushing the boundaries of practicality due to low light and atmospheric distortion.
3) 400/2.8 and 100-400/80-400 - bright, fast lens for low-light shooting at dawn or dusk. At 400mm on full frame, you'll be able to use your primary lens for most wildlife subjects, although you'll frequently be using it with a 1.4x or 2x teleconverter (not that that brings a huge image quality penalty with these lenses, although there may be more of a hit to AF).
4) 400/2.8 and 70-200/2.8 - same as option 3, but swapping out the 100-400 for a 70-200/2.8 for improved low-light performance. The 70-200 becomes a 98-280/4 with a 1.4x TC, reducing the gap between the upper end of the secondary lens and the focal length of the prime.
5) 180-400/4 or 200-400/4 - only goes up to 560/5.6, but leaves you with a main lens that will work in almost all situations, with little need to resort to a secondary lens.
(500/4 is really just a lighter and slightly shorter version of 600/4 and can probably be lumped into the same category)
What would be your choice of lens combination and why, and what environment would your choice be tailored towards? I'd personally lean towards 600mm for Arctic/Antarctic and 400/2.8 with 70-200 for Africa, but would be interested to see what others think.