The RF 100-300mm is not really a lens for birders, it’s too short. The 200-500mm with TCs goes from 700-1000mm if so desired. The 100-300mm with a 2x TC at 600mm f/5.6 is not as sharp as the bare RF 100-500mm. Before the 400mm DO II became available, I used the 300mm f/2.8 II for a few years. Excellent Lens that is, the longer ones are better for birding.
