Hi Tom, Sounds like you know what you're doing and you're going to have great fun!
The only reason for using DX was simply that the birds I was photographing filled the smaller frame nicely. I started out with (and still have) a D7000, I could have used that instead and got the same resolution, of course. Indeed, I used the 7000 last time out at the reserve, but with beanbag rather than tripod. Amazing when you think you've got x12 with the standard 35mm sensor/film camera, but x18 with the cropped DX sensor!
The Red Kite exercise was a Nikon training day hosted by an experienced wildlife pro. The instruction he gave on long lens use was superb. We looked at catching birds on the wing - trying to catch the bird at an angle such light catches the underside of the wings, compensating for shooting up at the sky, etc. We looked at catching birds cruising in front of trees with a slow shutter, 1/60 say, to get a sharp bird and blurred background. My best shot was taken with the birds swooping to pick food with talons primed and wings spread. I used my D4 most of the time as the birds filled the frame more or less, and light was poor so was shooting high iso. The number one lesson I learnt was preparation, panning ahead and with the bird in flight, having exposure compensation set, knowing exactly what the shot you want looks like so you know when to hit the shutter. This was my second session with him, first time we shot deer - much easier!!
I have the same tripod/head set up, by the way. I love the challenges shooting with long lenses, and get a real kick when everything coms together and you catch the shot you're after.