Getting great pictures of birds in flight is something that every avian photographer aspires to. There is both skill and luck involved in getting a crisp, well exposed and dynamic image which captures the essence of effortless freedom that is a bird on the wing. The skills are similar to those needed for other forms of action photography - anticipation, the ability to track your subject, and a keen sense of when to trip the shutter. As for luck, the barest fraction of a second can make the difference between a perfectly positioned wing, and one that's obscuring or shadowing the bird's head.
Equipment plays some part. A telephoto lens is essential. Fast and accurate auto focus is a big help, especially if the camera has the ability to accurately track moving subjects. It helps to practice with larger birds as their movements can be slower and more predictable, especially soon after take-off. These shots are at the low end of the scale when it comes to dynamic composition, but as opportunistic shots in passing, I can't complain.