First with the MB-D10 vertical grip, while it's a battery holder, for sure, it's far more than that, but for the moment, let's concentrate on the batteries. You will need an MS-D10 holder if you want to use it with AA batteries. Without that accessory ($26.95) you can't use AA batteries. I only know one person who went that route and he told me it was money wasted as he went through the batteries really fast. If you want to get to 8fps, I recommend using the EN-EL4a batteries. If your MB-D10 doesn't have the Nikon - BL-3 Battery Chamber Cover, you'll need that to use the EN-EL4a. It's $34.95. Of course, if you go for the EN-EL4a, you'll need the Nikon - MH-21 Quick Charger ($119.95).
If you go with the AA batteries, I know the Alkalines will give you enough power for 8fps, but I would also think about the Sanyo XX Eneloop 2500 mAh Low Discharge AA Batteries, but I'm not sure they will get you all the way to 8fps.
Don't forget all the other things the MB-D10 vertical grip can do for you. When you turn to portrait orientation you don't have to contort your right hand to reach the shutter release. Instead you use the shutter release on the grip, and you have both the main command and subcommand dials at your finger tips, and the AF-On button, etc. I use those controls often.
Now let's talk about birds in flight. You mentioned the shutter. Whether you're using manual mode, or aperture or shutter priority modes, keep your shutter speed at 1/500th or higher. I usually shoot with mine at 1/1000 or higher if possible. I'm usually in aperture priority, but I'm always keeping an eye on my shutter speed to make sure it doesn't drop too low. I use aperture priority so I can control my depth of field as necessary. I don't use manual as when I pan to keep the bird lined up, often there is a change in exposure settings needed and you need the camera to adjust for that itself.
For the D700 focusing I'd use Continuous-servo AF for my focus mode. My AF Area Mode would be set to Dynamic area AF. I would use either 9 or 21 points. I have found that most of the birds fly fairly regularly which I shoot, but just as important for the setting is that I've found that 51 points or 51 points-3D overwhelms the camera despite what Nikon thinks.