I love your Yellowstone images Gil
Especially the crop showing this one had been fossicking amongst the pine trees and still wears a few of their twigs.
Since you're going to locations such as this in conditions that will produce unusual opportunities but maybe test your endurance, usability ranks almost as high as technical capability.
It sounds like you're in for an enjoyable time, checking out the options. I don't know how long wintery conditions will remain in Yellowstone, but I'd love to see some 500mm PF images, when you get the chance.
Of course some taken with an 800mm f6.3 Z as others suggested in another thread would also be very welcome
