I was at the entrance to the Okefenokee in Fargo, GA last weekend, and snapped several images of this Great Blue Heron. He was a bit skittish but less so than some I've shot.
I shot with my recently purchased 500mm f/5.6 Pentax 67 lens. This huge beast is like 7 pounds, not including the camera! I hand-held these images and shot at f/5.6 or f/8 and 1/500 or 1/1000 as needed. The closer ones were with a #1 or #2 ext. tube.
This lens is quite remarkable and sharp considering its age. I used to have the 400mm f/4 Pentax 67 lens and I think this 500mm exceeds it slightly in performance at the widest aperture, but they are about the same stopped down.
Here are the images, shot on Portra 400: