This crow fact always interested me:

"Crows can remember faces (aka hold a grudge). A group of scientists conducted a study in which they wore masks while capturing and banding groups of crows. The crows learned to recognize the mask as a danger and would scold the person in the mask upon encountering them. Crows that hadn't encountered the mask started scolding as well. Over a five year period, the scolding doubled in frequency and spread almost a mile from the place of origin."