Revenge and Grudges
Crows are super crazy weird smart. And although it's hard to prove "super crazy weird" through science, researchers have at least been able to show that our onyx flying friends engage in super crazy weird smart behavior. Example A: recognizing the face that they hold a grudge for.
Scientists at the University of Washington had no idea what they were getting into when they started trapping crows for research. They noticed that the birds began harassing them whenever they stepped out of the office, regardless of what clothes they were wearing. So they experimented by wearing different masks when they trapped the birds. It turns out, when they would walk around campus later wearing the same masks, the birds once again wouldn't leave them alone. One researcher even put on a mask the crow crew had used five years ago for trapping and the Hitchcockian birds descended -- which implies that the older ones had let the word spread about which faces were good and bad [source: Pappas].
Admittedly, this grudge maybe isn't of the "I'll never forgive my cheating ex-boyfriend" variety (although I wouldn't put it past them). But it does imply that crows have an uncanny ability to recognize faces and to gossip with the best of 'em to let their community know who's a threat.