Well I would say it is more or less depends on the situation and kinds of birds you intend to photo. For offshore mudflap, even 600mm with a 2x is not enough for me. In case of forest birds on a trial hike,dense wood area, it makes much more sense to use something handheld instead of tripod mount, I've rather use a 400mm or sigma 50-500 for this kinds of shots. I am never fast enough to set my 600 on a tripod and capture those in dense wood, never stand still birds. Also a good set of camo clothes, and quiet movement is even more critical then the lens as not to scare the birds away in the first place. I often cover my camera, face and hands with camo net, which the birds tend to ignore me. Good things always come to those who wait.
