Photographing birds in flight is not the easiest thing you can do. Photographing birds in flight with a 600mm from a car is nearly impossible. While in Tanzania a few years ago I ended up using a bit of an odd technique when it comes to the smaller insect catching birds in particular bee eaters. They like to sit on sticks, watch take off catch an insect and land back in the same spot. So what I would do is focus on the bird while it was sitting on it’s branch, switch the lens to manual focus and wait for it to fly away. My step dad happily helped me out by telling me when the bird was about to come back to land. At that point I would hold down the shutter and hope that I got a good shot.
I think this result speaks for itself.