A scientific study provides evidence that crows are capable of recognizing threatening people and can even pass their concerns on to other crows. Researchers wearing rubber caveman masks trapped wild crows and then released them in the same area. Years later, people wearing the same masks near where the crows had been trapped were shrieked at and dive-bombed by crows.
The argument depends on the assumption that some of the crows that shrieked at and dive-bombed people wearing the masks were not among the crows that had been trapped.