I have a D800 and more than a few manual lens I use on it. For most things that don't move, there are absolutely no issues. I suspect the D600 has the focus indicator in the viewfinder:
> • <
It turns green when you hit the sweet spot!
The challenge is not the lens on the camera, but YOU trying to nail the focus! Birds in Flight? Good Luck. You could very well have a knack for this we are not yet aware of, but.... the likely hood of getting any really good images this way is slim to non.
You can of course, do excellent bird photography with manual focus lens. Follow my link here:
I have posted all sorts of images in this thread. It is a review of the Nikon 400mm f3.5 manual lens I use with my D800.
I also have a 500mm Reflex, 85mm, 35-105mm lens, all manual and all work really well. Nikon does have a compatibility list they publish on their site, if you are keenly interested in seeing what can and can't work.
I have purchased all these manual lens for fun... they were all well priced and I like to experiment. Where I am serious, I have AF as I can't see well, and trying anything like Manual focus on a moving target is an exercise in frustration and futility. Robin gives the same advise above, good advise.
Look at it like buying a Ferrari and then putting a set of GENERAL TIRE, regular sized all weather tires on it..... not the same as the Pirelli, low profile, summer racing tires! Same idea.
Maybe, let us know a budget.... I have gone through all this and have all sorts of permutation of lens that I have purchased. Happy to guide you further or give you an opinion or share an experience you might relate to.