A warning cry: To be published in Animal Behaviour, November issue
Authors: Kaeli Swift, John Marzluff
Many of us have often seen crows congregate noisily around their dead comrades. In their report that appears in the November issue of Animal Behaviour, Kaeli N Swift of the University of Washington and biologist John M Marzluff propose that crows pay attention to their dead as a way to gather information about threats to their own safety.
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, the authors say.
The researchers arrived at this conclusion after a series of experiments on the sidewalks of Seattle. As part of the experiment, Swift would sprinkle peanuts and cheese puffs on the ground and wait for crows to swoop in and feed on them. Next, a volunteer wearing a latex mask would approach them with a dead crow. The crows would invariably mob the volunteers.
If the volunteer carried a dead pigeon, however, the crows mobbed the person only about 40 per cent of the time. And if the volunteer stepped forward empty-handed, the crows just moved away and then returned to the food.
Though Swift 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. 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.
Adapted from NYT