Another route to 600mm
The Tamron 150~600 has generated quite a lot of excitrment, not surprisingly, but hre's another route to 600mm for a body with f/8 AF (5DIII in this case): 300/4L IS + Extemder 2× II. Not perfect, but not too heavy and AF can be workable if set up correctly. Here's a few from the wilds of Costa Rica, all hand-held.
All imgaes are cropped somewhat and resized to 1400×1050 for projection.