When I first came upon this scene it took me a few seconds before I convinced myself that it was not a person but a big bird "sitting" at the park bench. Although the pictures, taken with the digital zoom of a small camera left much to be desired, a friend was later able to identify the bird as a young red-tailed hawk (Buteo Jamaicensis).
The hawk let me approach it a little bit before flying to the top of a nearby tree, where a small—but, brave—bird started harassing it. Eventually, the hawk flew away followed by the small bird.
When I returned to the scene about 15 minutes later, I noticed a group of noisy crows flying from one tall tree to another. I suspected the hawk had come back and the crows were mobbing it. Sure enough, soon I spotted the hawk on a tree surrounded by the crows. I was able to get the picture below before the hawk left the scene again.
Most birds of prey, despite being skillful and feared hunters, never seem to get any peace from the smaller birds. At Words & Pictures, Roger B. mentions having watched a pair of choughs teasing a peregrine falcon. If you haven't the slightest notion what a chough is or how to pronounce it, read his post.