For serious travel and bird photography, I second fegari as I do use the f/2.8 300mm II with both series III extenders because less than that does not give the quality I want. For convenience, I used to travel with the 100-400mm L, but as written at length, in the SX50 threads, I can get along with the SX50, which under good light and static subjects outperforms the 100-400 on a 5DIII. The SX50 is a good back-up camera altogether as it goes from 24-1200mm equivalent and has a very good lens. The 1.4x on the 70-200mm f/4 is pretty good if you don't need the length, and I use to to use it a lot.