Crows can tell individual humans apart by their faces, and they hold grudges against people who have threatened them. This is not folklore. It has been tested in controlled experiments, and the results suggest crows keep track of friend and foe.

For a bird that lives right next to people in cities, farms and parks, this is a powerful survival tool. A crow that can tell a friendly neighbour from a threat is a crow that lives longer.

The best-known test came from biologist John Marzluff and his team at the University of Washington in the United States, beginning in 2006. They wanted to know whether wild American crows could recognise a specific person.

Researchers put on a rubber “caveman” mask and trapped, handled and released a group of crows on campus. Crows dislike being caught, so the mask became linked with a bad experience. The team then walked the same routes wearing the mask, and also wearing a different, neutral mask as a control.

The crows reacted very differently. They mobbed and scolded the person in the caveman mask, but mostly ignored the person in the neutral mask. Later tests used other masks and even different people wearing the same one, showing the birds were responding to the face and not to the individual’s height, clothes or walk.

The most surprising finding was that the hostility did not stay with the birds that were trapped. Over the following years, more and more crows scolded the caveman mask, including birds that had never been caught and were too young to have been around at the time.

The researchers concluded that crows learn about danger from each other. Some birds picked it up by watching a neighbour scold the masked person. Others likely learned it from parents and other family members. In this way, one unpleasant event could shape the behaviour of a whole local population.

The memory also lasted. Follow-up observations found crows still reacting to the mask years after the original trapping, and Marzluff has said grudges can last for many years.

The same research group later looked inside the birds’ brains. Crows that had been trained to link the mask with a bad experience were shown the mask again while their brain activity was measured with imaging, using a scanning method similar to a PET scan.

The results showed activity in brain regions tied to fear, learning and memory, including areas linked to emotion. In other words, seeing a hated face triggers the kind of response a person might feel on spotting someone who once frightened them.

This fits what scientists already knew about crows and their relatives, the corvids, which include ravens, jays and magpies. Their brains are small, but they pack in a very large number of neurons, and corvids are among the most intelligent birds studied.

Crows live in groups, stay in the same territory for years and depend on people for food in many places, from farm fields to rubbish bins. Knowing which humans are safe and which are dangerous helps them find meals and avoid harm.

Passing that knowledge along is a bonus. A crow does not have to be hurt to learn that someone is a threat, because it can learn from the alarm calls and behaviour of the birds around it. That kind of social learning is rare in the animal kingdom.

It also works in the other direction. Many people who feed crows report that the birds come to know them, and some crows even bring small gifts such as shiny objects, buttons or twigs to people who treat them well. These accounts are mostly anecdotal, but they match the research on recognition.

Crows do not just see people, they remember them. A single bad encounter can be learned by one bird, shared with others and carried on for years, which makes crows one of the few animals known to spread a grudge across generations.

The practical lesson is simple: if you want a crow on your side, treat it kindly, because it will probably remember.