Researchers trapped crows while wearing a particular mask. Years later, crows that were never trapped still mobbed anyone wearing that mask.
Source: Marzluff et al., Lasting recognition of threatening people by wild American crows, Animal Behaviour (2010). doi.org/10.1016/j.anbehav.2009.12.022
Image: Charles J. Sharp, CC BY-SA 4.0, via Wikimedia Commons
