OK I would read up in the camera manual about focus modes.
You should probably be in AF-C. If you focus using only the AF-ON button then you probably already are in AF-C. Then experiment with 9 point, 21 point or 51 point dynamic AF. The manual recommends 51 point for birds but others here have said that this mode slows the AF down. Dynamic area AF allows the camera to continue focusing using the surrounding points if your subject leaves the selected point momentarily....which happens when you are trying to track a bird in flight.
Regarding exposure - if shooting against the sky, the underside of the bird is usually in shade and the brightness of the sky will tend to bias the camera into underexposing the bird so consider setting a 1 to 2 stop overexposure if using matrix metering, or use centre weighted metering with +1/3 to +2/3 stop exposure compensation. Shooting against blue sky is much better than cloudy or overcast skies where greater correction may be needed. If the birds have light or white plumage and are brighter than the sky (e.g. seabirds) or if shooting against darker backgrounds such as foliage then the advice above will overexpose them so be prudent and set up your exposure compensation for the conditions on the day. Take a few test shots, even if not in focus and adjust if needed. Histogram could be useful for this.
Regarding ISO : Not sure what your appetite for high ISO noise is but mine is about ISO 2000 on the D800. So I have a bright-handheld shooting mode set up in the menu banks in which Auto-ISO is set up with a ISO 2000 upper limit and a shutter speed of at least reciprocal of focal length.
Aperture - set to preference. 70-200 with converters means you could be anywhere out to 400mm so depth of field could get too narrow to encompass the combination of nearside and farside wings plus a bit of misfocus. Suggest to shoot at about f5.6 to f8. Auto ISO settings will take care of the shutter speed.
VR - your lens may have a panning or "active" mode in which case this can be used. I suggest you don't use "normal" VR. If shutter speeds are routinely higher than about 1/500 then VR may not help and you can consider turning it off.
James mentioned the 9-, 21- or 51-point settings and the 3D Auto-area. IMO the best mode depends on the background. 3D tracking is good if I can first focus on the bird against a clear sky. When the focus system learns the color of my target, it can track the target even against foliage or the like. But it's almost impossible to use the 3D Auto-area if the bird is already against a textured background. Focus with AF-ON and AF-C is the way to go.
Hi, Rich! These are all excellent suggestions! My suggestion is to use your camera hand-held because a tripod or monopod may be too constrictive and you might trip over your tripod leg as you're panning. Apropos "panning": in addition to fast shutter speeds (if you want to freeze the bird in flight), you should practice your panning technique. Start by photographing a bicycle, a skateboarder, or a car as they ride/drive past you. Once you feel that you've got the technique down (which might involve twisting your body), then try it with birds. This, combined with the suggested settings, should result in some really great photographs of birds in flight.
Hope this helps!
_________________________________ The camera doesn't make a bit of difference. All of them can record what you are seeing. But, you have to SEE. Ernst Haas, 1985