Well, it really depends how close you are and how big the flock is. What field of view do you need?
Sometimes a 200mm is all you need for a flock, maybe even less if it's a big flock or you're up close. Or if it's distant, you might need 400 or 600.
With money as no object, really the best lenses are 70-200/2.8 (or 200/2), 300/2.8, 400/2.8, 500/4, and 600/4, or even the 800/5.6. The other zooms like the 180-400/4 or 100-300/2.8 (replaces the 300/2.8 fixed) would also be excellent choices. These are all native Nikon glass. Maybe the new Sigma 70-200/2.8, but I'm not positive on that compared to the Nikon 2.8E.