Tokina at-x pro 300mm f2.8 on D800
I think you will find 300mm is too short for birds, unless you get very close. Here us an uncropped d800e picture, followed by a 387 square pixel crop from it, of a Ladder Backed Woodpecker. This is using a Sigma 50-500mm OS, hand held.
You might consider this lens. It is not quite as good as a Nikon 500mm f4 vr, but is very good, and much more practical, because it weighs and costs about one 5th.
www.pbase.com/bertramm pbase & dpreview supporter