Most of us have seen it one time or another. A larger bird being mobbed by smaller birds- Red-tails being harassed by ravens, ravens being harassed by black birds. It happens all of the time. It is the preemptive strike of the birding world. And it often happens between different species of raptors.
The Kestrel (smaller of the two birds) while small, is very territorial, and will defend its territory quite vocally and physically against larger predators. While they don’t really do any damage (except the theft of a feather or two) their persistence can often drive other birds away. While they are smaller and not as strong as many of the birds they sass, they are much more maneuverable and are not in too much danger, as long as they have a little care.
I managed to capture this shot while I was driving down one of my local country highways, and I paused to take a few images of this female kestrel (you can tell from the coloring of her feathers). It wasn’t until I got out of my car that I heard her cries and realized that she was protecting her territory from a larger intruder. I studied her cycle of diving and was able to time this shot as she neared the Red-tailed hawks head.
I of course used a long telephoto lens to capture this image from a considerable distance away. Shutter speed was maximized to freeze the fast paced movements. Everything else was arranged to compensate.