When I lived in Connecticut, I once heard a ruckus in the backyard. I looked out my window to see a crow standing on the roof of my toolshed. In the trees surrounding him were five grackles. The crow could only face one of them at a time. Whichever way he looked, the grackle who was directly behind him would leave his perch and swoop in at the crow. The crow would hop around and change his orientation to face that assailant. As soon as he did so, the approaching grackle would pull out of his dive, while the grackle who was now behind the crow would swoop in towards him.
After several minutes of this, the crow took off from the roof. All five grackles followed him, harassing him, no doubt, until he was out of their territory. Is there a "Driving off Crows" School were they go to learn that sort of tactic?