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 assumption that even in places where crows have never been captured, most crows will shriek at and dive-bomb people wearing caveman masks is required by the argument.