Studies have shown that crows can learn to recognize human faces and hold those faces (and any grudges associated with them) for a long time. Researchers captured 12 wild American crows while wearing particular mask of a human face.
For four weeks following their capture, they fed and cared for the crows while wearing a different mask. One by one, they took the crows and presented them with one of the two faces or an empty room. They gave the crows a chemical that functions as a sort of dye or marker. It would be taken in by whichever part of the brain became active. They crows would then be examined in a PET scanner.
From the images and the crows behavior, it was evident that the crows recognized both masks, and treated the capturing mask as a threat and the feeding mask with relative affection. They did so with the same region of their brains that humans do when they process images of faces and associate them with relevant emotions.