I have been playing with bean bag versus window mount for telephoto shots from a vehicle blind. This hawk was taken on a BLUB (Big Lens Ultimate Bag). The bag is filled with beans to provide weight, yet not be too heavy and bulky like sand would be.
I used my left hand to dampen lens vibration and held my breathe and gradually let it out as if I was firing a rifle.
The next two shots are using a Wimberley II head on a Gitzo tripod.
The Next two pictures are doing hand held 200-400 through a window!
the Sparrow shot showed very little movement, yet the rear leg is not in focus due to the short depth of field from using f5 to keep shutter speed as high as possible for the hand hold.