1. Prefocus: Obviously you can't prefocus anywhere in the sky. If you know that a bird is going to land on a perch or going to fly infront of you, then you estimate the distance and manually focus in the area. Without any reference point, it would not be easy to prefocus.

2. Focus Assist: When you are trying to click a bird, just help the camera acquire focus quickly by turning the focus ring manually to bring in near focus. The AF will then immediately lock into it.

3. When you are shooting against the sky, the amount of positive exposure compensation you have to dial in depends upon how big the bird is in the frame. It can be +2/3 to +1 depending upon situation. If time permits, then you can give a quick glance at your histogram to know. When you have more experience, you can try to go manual by learning exposure theory. http://www.indiawilds.com/forums/showthread.php?t=1970

4. Panning is for lower shutter speeds. If the bird is flying fast, then you can get panning effect in slightly higher shutter speeds.