I spent some time on Saturday morning at the bird blind in Charro Ranch Park, Dripping Springs, getting some practice handling really long lenses. There were plenty of birds to point the camera at though most were of the common-or-garden variety such as this cardinal.
I was shooting with various combinations of the 300mm f/4, 100-400mm f/5.0-6.3 and the MC-20 2x teleconverter, all handheld. This particular image was taken with the 300mm and teleconverter for a 35mm equivalent focal length of 1200mm. That combination definitely gave the best sharpness but I found it tricky to acquire the target (about 25 feet away in this case) after lifting the camera to my eye due to the tiny field of view of the very long lens. Using the zoom, I could zoom out, figure out where the bird was, and zoom in to recompose without taking the camera from my eye. The downside here, of course, was that the birds often took flight in the time it took me to set up the framing. No doubt more practice will help with both of these, though, and I did find things getting easier the more I shot.
For the technically inclined...