Once you've lost focus on a bird in flight and the lens starts hunting, you're probably not going to get it back by trying to follow the bird. Try pointing the camera at a distant point on the ground and focusing, so the camera's focus is set for a distant target. Then aim at the bird and when the bird is under your focus point, you should be able to get it in focus and get your shot. The 55-300 does not have a focus limiter switch (which would help, if the lens had it). There is a learning curve for this type of shooting, so I think you will find that with more experience you will be better able to get those shots.