Why do birds fly in strange, orbiting circles?

While driving to the bust stop this morning, my mother and I saw an interesting phenomenon – birds, which appeared to be grackles, were circling around in the air in a strange pattern. They didn’t appear to be going after anything in particular, such as food. They simply formed a tightly packed, sphere-shaped flock and circled over the road ahead of us.

I have seen them do this before when chasing away an osprey, and at other times for no known reason at all. I live in Southern Florida. Does anyone know why they behave like this?