Kevin Chow , Mar 10, 2011; 12:32 p.m.
Yep, this is the 3D tracking AF mode. In the D7000 (and probably by extension, to the D300 as well), you stick part of the bird in the selected focus point (which you can move about as you would normally), half-depress the shutter release to the focus detent and then hold it down while tracking. It uses some image pattern recognition algorithms to try and keep whatever you focussed on as the focus point, so as the bird or object wanders around the frame, the AF point tries to follow it.
It works fine against a clear sky, but if you're shooting against very contrasty backgrounds, it can get lost.
I use a combination of 3D tracking and 9-point AF when shooting birds. 3D tracking on the '7000 works pretty quickly, and unless you're using a fast lens that can keep up, it can hunt. It works fine with my 70-300 VR, but the AF in my Sigma 150-500 is too leisurely, so it doesn't work so well - in that case I just use single or 9-point AF. In recent experience shooting waterfowl with the sigmonster, I stuck mostly to 9-point AF - partly because of its AF speed, but mostly because most of the time, birds were far away enough to give me time to pan around and take my time.