Easily my favorite for most birding is the 600mm f/4.....BUT...I do use at least three other lenses regularly in my birding situations: 300mm f/4L, the 100-400 f/4.5-5.6L, and the 70-200 f/2.8L. It's really more dependent on the environments that I'm shooting in at the time. My favoriite BIF lens is the 300mm f/4 or the 70-200 2.8. My favorite "general purpose" is the 100-400.