Crows remember the faces of threatening humans and often react by scolding and bringing in others to mob the perceived miscreant, according to a new study published in the latest Proceedings of the Royal Society B.
Since the mob members also then indirectly learn about the threatening person, the findings demonstrate how just a single crow's bad experience with a particular human can spread information about this individual throughout entire crow communities.
Given that crows have impressive memories, people who ruffle the feathers of these birds could experience years of retribution.
Bothered crows may at first "give harsh calls, which we call 'scolds' that attract other crows who are nearby to join in the mob," according to study co-author John Marzluff. "The mob of two to 15 birds hounds us, sometimes diving from the sky to within a few meters or less -- This pursuit lasts about 100 meters (328 feet) as we walk away."
Marzluff is a professor at the University of Washington's School of Forest Resources. For the study, he and colleagues Heather Cornell and Shannon Pecoraro exposed wild crows to a novel "dangerous face" by wearing a unique mask as they trapped, banded and released seven to 15 birds at five study sites near Seattle.
The released birds immediately scolded the mask wearer. Hearing the racket, other crows joined, forming an angry mob.
When the researchers later put on other masks while traveling to different areas, crows that were never captured immediately recognized the "dangerous face," illustrating how these birds learned through social means and not as a result of direct experience. Both relatives and strangers joined in the scolding and mobbing, which could occur over a mile away from the original incident.
Once such a face is locked into a crow's memory, it's likely there for good.
"Our study shows the memory lasts at least five years and counting," Marzluff said. "Individual crows that are adults can live 15-40 years in the wild (most die when young, but those that make it to adulthood can live a long time) and they probably remember important associations they have formed for much of their lives."
Prior research demonstrates that crows are particularly intelligent birds.
"Others have shown that some crows make and use tools, forecast future events, understand what other animals know, and -- in our case -- learn from individual experience as well as by observing parents and peers," Marzluff explained. "These are all advanced cognitive tasks shown by only a few animals."
He suspects other social, long-lived species that live closely with humans might also share information in a similar manner. Possibilities include animals such as coyotes, raccoons, gulls, pigeons and rats. All could practice a combination of social and trial and error learning. The latter provides the most accurate information, but it is clearly riskier than indirect social learning.
In the animal kingdom, humans have a language advantage because we can just verbally warn others about dangers. Vocalizations do not appear to be enough for crows, which appear to require visual observation for the information to sink in.
Anne Clark, an associate professor in the department of Biological Sciences at Binghamton University, told Discovery News that this new study "suggests importantly how much long-term studies of individuals have to contribute to our understanding of adaptive social learning."
She added, "I doubt that anyone working with crows will be surprised by the results suggesting several routes of social transmission, but this kind of formal, empirical testing is much needed."
Kevin McGowan, an instructor at the Cornell Lab of Ornithology, echoed Clark's comments, but said the findings might surprise "anyone who thinks animals only learn by direct experience."
"Social animals are social for lots of very good reasons," McGowan continued. "This study demonstrates one of the more subtle ways that animals benefit from interactions with other members of their own species."