Thanks, Tom. I have an earlier version (D vs. G) with a different optical formula. I haven't tried yet to shoot smaller birds; I might try it with a TC-14E on a D7200 to get more reach.
The 400 2.8 is far and away my favorite lens. When it gets the picture right, it is so right! It has two drawbacks - it's too heavy and it's getting a bit long in the tooth (as is its owner...) and there are worries about something breaking. It works very well with a 1.4 teleconverter on a full frame body but never tried it with a cropped body.
Smaller birds (i.e. less than six or so inches long) are really only manageable for me when they stay perched or proximate to that perch. Once aloft, it's a crapshoot. I wish I could fly and go up to the treetops and shoot warblers in the spring - maybe in the next lifetime.
Now, to master the D810 or at least manage it better.
And then there's the software. I could go on but might lose my sense of place (and the reader's attention) in this rambling post.