One of my first outings with my 600 f/4 was to photograph sandhill cranes. You'll quickly learn that for birds in flight you need to start tracking them at a long distance, and then give up a little quicker as they get close. Add a teleconverter if you want some real fun.
For small birds, the tiny DOF of a 600mm lens takes some adjustment. It's just a fraction of an inch at 20 feet.
Be sure you have a good strap to carry the monster. I use a Black Rapid strap with a quick release. I walk longer distances with the lens off the camera.
My first images with the D800E and 600 f/4 were jaw dropping sharp. Even at crops of 100% I had images that could be posted on the web.
If you have the Nikon teleconverters, I'm sure you'll give them a try. I find the 1.4 teleconverter is quite good. And the field of view with the 2.0 teleconverter makes shooting through a straw a reality.