Ms. Swift then ran more tests to see how much of an impression the dead crows made on the live ones. Because crows can tell individual humans apart by their faces, she had her volunteers wear latex masks. Even though she used a rotating crew of volunteers, each group of crows would see the same face throughout the trial. She had them return to the feeding site once a week to see how the crows responded.

“It’s a very Hannibal Lecter thing — it looks like you cut someone’s face off and are wearing it,” said Ms. Swift, who spent a lot of time reassuring Seattle residents that she was actually doing science. “A lot of people would say, ‘I don’t care what you say, I’m calling the police.’ ”

Up to six weeks later many birds still scolded the visitors even when they approached with nothing in their hands. Volunteers wearing unfamiliar masks, on the other hand, were scolded significantly less often.

Ms. Swift found more signs that dead crows left a strong impression on living ones. In the days after seeing a volunteer with a dead crow, birds took significantly longer to approach food. The sight of a dead pigeon had no such effect.

In their report, which appears in the November issue of Animal Behaviour, Ms. Swift and Dr. Marzluff propose that crows pay careful attention to their dead as a way to gather information about threats to their own safety. “It’s a long-term learning opportunity,” Ms. Swift said. “Knowing that you need to be wary in a particular place — that’s valuable.”

The presence of a dead crow could tell other crows that a particular place is dangerous and should be visited with caution. The loud calls the birds make could be a way to share information with the rest of their group.

“Work like this helps to remind us of the cognitive complexity that exists in animals other than humans,” said Teresa Iglesias, an evolutionary biologist affiliated with Australian National University who was not involved in the study.