Birds of a feather not only flock together, in the case of the American crow it seems they are also into group learning.
A five-year study of crows living near Seattle in Washington State show the birds can remember a "dangerous human" and are able to share their knowledge of the learned danger with their offspring and other crows.
It is a trait, says co-author Professor John Marzluff of the University of Washington, that can help species successfully adapt to, and co-evolve with, humans.
"The behaviour of individual people towards animals is often changing," says Marzluff.
"Because human actions often threaten animals, learning socially about individual people's habits would be advantageous."
Marzluff says the study, published today in Proceedings of the Royal Society B, was inspired by earlier research with crows on the university campus.
"I felt like they recognized us," he says.
To test his theory, two researchers, each wearing an identical "dangerous" mask, trapped, banded and released 7 to 15 birds at five different sites near Seattle.
To determine the impact of the capture on the crow population, over the next five years, observations were made about the birds' behaviour by people walking a designated route that included a trapping site.
These observers either wore a so-called neutral mask or one of the "dangerous" masks worn during the initial trapping event.
Within the first two weeks after trapping, an average of 26 per cent of crows encountered scolded the person wearing the dangerous mask.
Scolding, says Mazluff, is a harsh alarm kaw directed repeatedly at the threatening person accompanied by agitated wing and tail flicking. It is often accompanied by mobbing, where more than one crow jointly scolds.
After 1.25 years, 30.4 per cent of crows encountered by people wearing the dangerous mask scolded consistently, while that figure more than doubled to around 66 per cent almost three years after the initial trapping.
Marzluff says the area over which the awareness of the threat had spread also grew significantly during the study. Significantly, during the same timeframe, there was no change in the rate of scolding towards the person wearing the neutral mask.
He says their work shows the knowledge of the threat is passed on between peers and from parent to child.
"Crows recruit and tolerate others of their own and different species in mobs that form around dangerous people," he says.
"This social tolerance could allow naïve crows to learn about dangerous situations, locations and individual humans."
Marzuff says he had thought the memory of the threat would lose its potency, but instead was "increasing in strength now five years later".
"They hadn't seen me for a year with the mask on and when I walked out of the office they immediately scolded me," he says.
"I believe the fact that scolding 'works' in our system is somewhat self perpetuating as well because we move on down the trail as we are scolded.
"This might reinforce the behaviour as the scolding birds see a payoff (our departure) in response to their efforts."
Marzuff says the crows are very precise in their identification of the dangerous human.
He says the initial "dangerous" mask was a caveman's face with a mask of former U.S. vice-president Dick Cheney the neutral or control face.
However the team made six other masks — combinations of male, female, Caucasian and Asian — which were used at various sites as a dangerous face or a neutral face.
These looked a lot more realistic and similar to each other, yet the crows were "very, very good" at identifying which person was dangerous.
Professor Ken Cheng, of the Department of Biological Sciences at Macquarie University, says he is not surprised by the finding as birds are known to be smart animals. He says the crow belongs to the corvid family, which is commonly referred to as the "avian Einstein".
"If you can learn which of the animals are dangerous without having to face the danger you are going to benefit," says Cheng, who was not part of this study.
"It makes good survival sense to learn socially as it can be rather dangerous to test the danger yourself."
Cheng says although similar work on Australian native birds had not been done, the study cites a paper on social learning by Australian miner birds by University of Newcastle researcher Dr. Andrea Griffin.
Cheng says the paper is especially interesting because it is based in the urban ecology, which is a new niche for many animals.
"It is on the messy side because it is a field study, but that is inevitable and it more than makes up for it in the reasonable and careful inferences it draws about the data," he says.