1200mm is long, but not that ridiculous. Certainly not nearly as insane as the extreme size and weight would suggest. I don't bird right now much simply because I have not modified the mount and remove the aperture lever of my lens to not make contact with my pins of my body. But when I did 1200mm (600mm, 2X) was very useful and not even that long for wild birds. Even then I still have to stock them for a long time since they are just so skittish. I used it a lot for an insurance shot, I could get it with 1200mm and then try and get it with 600mm. Sometimes I can get close enough sometimes not.

Weight and size? It's not exactly meant for everyone, but I for one do not mind. It serves a purpose, and that is having an extremely wide aperture for the focal length. At least for me.

Not that I'm a world class birder, but one can dream right?!