Hello Rich,
Recently witnessed a single large crow having a go at a buzzard, and I was quite amazed at the determination of the crow. The buzzard had to get into a thick pine tree, to try to get away from the crow. The crow didn't stop however, and still persisted, finally driving the buzzard off.
I've seen 2 crows going after a kestrel. One crow was playing 'Top Gun' and flying about 15 feet above its partner, who was getting stuck in. The Kestrel easily out-turned the crow, but the crow kept going, for a good minute or more. At this point, 'Top Gun' dived in, and took over the attack, the other crow flying up, out of the melee. After about another minute, the positions were reversed. This kept going for a good 5 minutes.
Ian