A new study has revealed that wild crows are capable of recognizing individual human faces and they can hold a grudge for years.
This ability - which may also exist in other wild animals - highlights how carefully some animals monitor the humans with whom they share living space.
For the study, John Marzluff at the University of Washington in Seattle and colleagues donned a rubber caveman mask and then captured and banded wild American crows.
Whenever a person wearing the same mask approached those crows later, the birds scolded them loudly.
On the other hand, they ignored the same person wearing a mask of former US Vice-President Dick Cheney, which had never been worn during banding.
"Most of the time you walk right up to them and they don't care at all," New Scientist quoted Marzluff a saying.
The birds' antipathy to the caveman mask has lasted more than three years, even though the crows have had no further bad experiences with people wearing it.
The crows responded less strongly to other details of a person's dress, such as the presence of a hat or a coloured armband.
In a second experiment, Marzluff's team prepared six masks from casts of people's faces, then wore different masks to capture crows in each of four locations.
In each case, they found, the crows recognized and scolded whichever mask they had seen when they were captured, and ignored the others.
Doug Levey of the University of Florida in Gainesville, who was not part of the team, said that this shows that crows pay close attention to humans, noting which individuals pose a threat and which do not.
The study has been published in the Journal Animal Behaviour.