Conversely, you might want to stop down the lens a bit. I find the DoF can be very shallow, and increasing the DoF can help cover any focus errors.
I try to shoot at around f/8 for bif for the reason popo stated above.
You may very well have to crank up your iso to get your shutter speeds up.
Try using 21-point AF, and for myself I avoid using a tripod for all my wildlife shooting as to me anyway, is a pain in the ass and have missed some great photo opps.
Not sure what birds you intend on shooting, but the 70-200 in most cases you will find too short. Unless you are after local ducks and gulls.
For the money you would spend on the Tamron 70-200 I would instead look at the Sigma 120-400 or the 150-500. Not fast lenses but you will get the reach. For the money the Nikon afs70-300vr is a great lense.
Practice on your panning very important, try moving your hips only to pan not your arms or upper body.