I went to Yellowstone n May 2010, just as the park was re opening. I took the 400 F5.6, 70-200 F4 neither have image stabilisation, had no problems with birds in flight. I also took 2 bodies as well 7d and 40d as well as 10-20/24-70 for landscape shots. The 400 F5.6 trounces the 100-400 for image quality - tested it against my friends copy who after seeing the difference he bought a 400 F5.6 as well. I got some good bison, elk, moose and bear as well as birds.
The 70-300L as far as I know is weather sealed so that would be a good one to take as well as having image stabilisation. If you rent it ask it if it comes with a tripod collar as I dont think the standard model does.
« Last Edit: January 20, 2012, 10:11:55 AM by docsavage123 »
Canon 7D, 5D Classic, T2i, 100-400 F4.5-5.6L IS, 400mm F5.6L, 70-200 F4L, 1.4 EXT ii, EF100 F2.8 Macro, EF50mm F1.8 ii, 40mm Pancake, Sigma 24-70EX DG F2.8, Sigma 17-35 F2.8-4 HSM, Sigma 18-50 F2.8-4 OS , EX430 II. Red Snapper tripod.