Once one of my Photography Instructor asked me a simple question, “Which one is better, a watch that is stopped or a watch that always runs 10 minutes late?” The answer is, always prefer the stopped watch as its gives correct time at least twice a day. In case focusing for a moving element I always remember the same. There are different types of birds in terms of their flying speed and size. The Egrets are medium sized (Though they have thin neck that makes it difficult to focus their eyes) and their quite slow. The falcons are extremely fast and some birds fly very infrequently. Some birds fly and stop in the air for sometime to see everything around and then proceed. Some birds do walks/runs very fast with the wings spread, like jungle foul.
Here are my options,
1. Use Predictive focus and follow the bird and on the decisive moment press the shutter. The disadvantage is you should know the bird’s behavior completely, otherwise the decisive moment will after the bird will go out of range. Good option for egrets and herons etc.
2. Focus to place where you know the bird will come or fly through. Wait for the bird to come and press the shutter at right time. Disadvantage you have to keep your eye fixed with viewfinder and even missing a second would be quite costly.
3. Let say the bird is flying 20 feet from where you are, focus approximately around 20 feet and wait for the bird to come. If you focus anywhere else the auto focus will take time to focus the bird. Let say the bird is now flying around 25 feet away, so your camera will hunt from 20 feet to 25 feet, so the chances of getting a sharp picture is more. But if you focus either nearby, say 5 feet or at infinity then the auto focus will take time for hunting. So there is high possibility of missing the moment.
4. Use your instinct and just click at the right time, no rule can describe this.
5. Get Closes to your subject as much as possible. Otherwise chances are there that you may loose the sharpness. Try to focus at eye and nose. Using, if light permits, narrower aperture helps. Most of the time to track down a bird in motion, I use f/9 or f/8. For other Animal, as there is a inch gap between eaye and nose, increasing the aperture helps in getting sharp picture
6. Use Predictive focus, for Canon camera AI Servo and AI Focus. AI Servo should be used when your subject is in motion continuously and AI Focus for subject that might not on move continuously.
7. Last Point, I practice a lot, I have rubber bird, I have fixed it somewhat like a swing and keep tracking it with my camera. That’s helps a lot.