UW biologists Kaeli Swift and John Marzluff were intrigued by the way crows seem to congregate noisily around dead comrades. So they decided to run an experiment on the phenomenon.
In recent years, a peculiar sort of public performance has taken place periodically on the sidewalks of Seattle.
It begins with a woman named Kaeli N. Swift sprinkling peanuts and cheese puffs on the ground. Crows swoop in to feed on the snacks. While Swift observes the birds from a distance, notebook in hand, another person walks up to the birds, wearing a latex mask and a sign that reads “UW CROW STUDY.” In the accomplice’s hands is a taxidermied crow, presented like a tray of hors d’oeuvres.
This performance is not surreal street theater, but an experiment designed to explore a deep biological question: What do crows understand about death?
Swift has been running this experiment as part of her doctoral research at the University of Washington, under the guidance of John M. Marzluff, a biologist. Marzluff and other experts on crow behavior have long been intrigued by the way the birds seem to congregate noisily around dead comrades. Marzluff has witnessed these gatherings many times himself, and has heard similar stories from other people.
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“Whenever I give a talk about crows, there’s always someone who says, ‘Well, what about this?’” he said.
Marzluff and Swift decided to bring some scientific rigor to these stories. They wanted to determine whether a dead crow really does prompt a distinctive response from living crows and, if so, what the purpose of the large, noisy gatherings might be.
To run the experiment, Swift began by delivering food to a particular spot each day so the crows learned to congregate there to eat. Then one of her volunteers would approach the feast with a dead crow, and Swift observed how the birds reacted.
Almost every time, the crows mobbed the corpse-bearing volunteers. Swift is eternally grateful to her volunteers that they didn’t abandon the research at that point.
“If you’ve ever been divebombed by a crow, it’s really terrifying,” she said.
If the volunteer carried a dead pigeon, however, the crows mobbed the person only about 40 percent of the time. And if the volunteer stepped forward empty-handed, the crows just moved away until the coast was clear and then returned to the food.
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 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.
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, Swift and 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,” 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.
That’s not to say every animal pays attention to its dead, however. In fact, the club is fairly exclusive, including species such as chimpanzees, elephants, dolphins and relatives of crows known as scrub jays.
“It’s pretty consistently animals that live in social groups and are known for having more advanced cognitive skills,” Swift said. “It’s amazing to think a crow — a bird — is doing something like this that so few other animals are doing that we know.”