Patterns, Arcs, and Circles
Contrary to what some people believe birds don’t generally fly for the fun of it. As far as I can tell and all things being equal, they would just as soon remain stationary. Food, shelter, saftey, and procreation are a birds motivation in life and all of their flying time serves one of those needs. They are creatures of habit and are always going to perform repeating patterns in flight. They like to fly along a shore line for example, and you will often see birds carry twigs on the exact same route when nest building. I know of one osprey that has a favorite fishing hole and most every day that bird can be found sitting on a branch above the hole waiting for the right fish to come along then swoops down to grab it.
Generally, they will also take off and land into the wind. That tells us that most birds very seldom fly in straight lines. All birds are going to circle around the nest at some point or follow the curves in the shoreline or river bed looking for food. Soaring birds always make circular patterns. Use this to your advantage, set up and track from profile all the way to head on and get a series of images. Many cameras acquire and track focus much better when the subject is moving across the field of view rather than straight at you. Using that technique will make things easier for the camera to do its job.