Can crows remember human faces? Scientists in the United States wore a special mask when they caught seven wild crows. Later, they walked around wearing the same mask. Many crows made loud calls at them.

The crows did not react much to a different mask. This anger lasted for at least two years. Even crows that were never caught joined in. They learned from other crows that the face was dangerous.

Crows can recognize human faces, and they remember people who treat them badly. Researchers at the University of Washington wore a certain mask while they caught seven crows on their campus. Later, when people walked around in the same mask, many crows scolded them loudly and gathered in groups.

However, the birds hardly reacted to a different mask that had not been used for catching them. This reaction continued for at least two years. Over time, the number of crows that scolded the mask actually grew. Some of them had never been caught, and some were young birds born after the experiment. The researchers concluded that crows learn about dangerous people by watching other crows, a process called social learning.

Can wild birds remember one particular human face? To find out, John Marzluff and his colleagues at the University of Washington put on a distinctive "dangerous" mask while they trapped and banded seven American crows on campus. Later, volunteers walked through the area wearing either that mask or a neutral one. The crows scolded and mobbed people in the dangerous mask and mostly ignored the neutral one. It made no difference who was behind the mask, and the reaction lasted at least two years.

Instead of fading, the number of scolding crows grew. It came to include birds that had never been caught, and even some that hatched after the trapping. A 2012 follow-up concluded that the knowledge had spread through social learning, both between peers and from parents to their young.

Many of the birds were never individually identified, so the exact routes it took are hard to trace, and a mask is only a stand-in for a real face. The team also looked inside the brain, using PET scans on twelve crows. When those birds viewed the threatening face, a region resembling the mammalian amygdala, an area linked to fear learning, became active.