Crows can recognise individual human faces and hold grudges

Research by John Marzluff at the University of Washington showed that crows can recognise and remember individual human faces for years. In experiments where researchers wearing specific masks trapped and banded crows, the birds subsequently dive-bombed and scolded anyone wearing those masks on campus — even years later — while ignoring people in other masks. More remarkably, un-banded crows who had witnessed the trapping — but not experienced it — also learned to mob the 'dangerous' face, suggesting social learning of threat information across the flock.

Source: Marzluff et al., Animal Cognition (2010)