My own experience with continuous focus with birds in flight on my D7000 is that success depends on the background. Against the sky it works well. Most of these birds fly fairly low most of the time, though, so you get a mixture of trees, sky, more trees, even buildings in the background, and the bird doesn't stand out well enough for the focus system to follow. It keeps grabbing other features in the viewfinder.

Like Jeff, I shoot birds in single-focus, but when I shot the Blue Angels air show last year (unfortunately the last air show I've been able to attend since the budget crap shut everybody down) I shot in continuous-focus, and out of over 1,000 frames in two days, I had maybe two that weren't quite right, and those two I'm not sure weren't motion blur rather than OOF.