Hi Kipper,
For flight shots, 300mm to 400mm is enough. Plus the lens cannot be too heavy or too light. If it is too heavy, your arms will tire easily and becomes shaky. Too light, you can easily move the camera when pressing shutter button.
For stationary stots, the longer the better on strong solid tripod.
Lighting is very important. Poor light = Poor image being recorded on the sensor. But of course, lighting is of no use if we can get close to the bird enough. They simply fly off and we are back to square one. If we can't get close enough, the subject recorded are like specks of pixels that are not at all useful.
Bird photography is not for the faint hearted.
regards,
Arthur