It is well established that crows are highly intelligent. They are known to use tools in the wild, and have remarkably sophisticated tool-making abilities. In the lab, they can solve complex problems, such as using three tools in sequence to obtain food. Other members of the Corvid family have equally amazing cognitive skills. The Clark's nutcracker, for example, caches up to 100,000 nuts in dozens of different locations at the end of spring, and can find them all again up to nine months later, even if they are covered with snow. They have even evolved clever strategies to guard their caches - if, while storing nuts, they are aware of being watched, they will return some time later to retrieve the nuts and bury them again elsewhere.  

The idea of using crows to find the world's most wanted man was based on the work of John Marzluff, a wildlife biologist at the University of Washington who has been studying crow behaviour for over 20 years. Working with a population of wild American crows on the university campus in Seattle, Marzluff and his colleagues noticed that birds which they had previously captured seemed to be wary of them and were harder to catch.

The researchers therefore decided to investigate the possibility that crows can recognize human faces, and devised a relatively experiment using rubber masks. They went out on campus and in the surrounding areas wearing either a 'caveman' mask or a Dick Cheney mask. Those who wore the caveman mask caught and banded between 7 and 15 crows on each excursion, but those who wore the Dick Cheney mask did not. 

In the following months, they went out wearing the same masks, walking around  the university campus in pre-determined routes without bothering the crows. They also recruited volunteers to do the same. The crows consistently harassed anyone they saw wearing the caveman mask, scolding them with loud squawks and even mobbing them.

This happened regardless of the size, sex or walking style of the person wearing the mask., and even when the mask was partly hidden under a hat or worn upside down. They were, however, indifferent to the neutral mask - when they saw both masks simultaneously, they would ignore the person wearing wearing it, and instead follow the person wearing the caveman mask and scold them aggressively.

Evidently, the birds had perceived the caveman mask as threatening during the initial part of the experiment, and had remembered it.

What's more, their memory of the mask was persistent - nearly three years later, they continued to attack anyone who wore it. Marzluff says that he has been scolded by far more birds than had been originally trapped, suggesting that they not only recognized the mask, but had transmitted the information to their offspring and to other birds in the flock.

Marzluff and his colleagues published  their findings about a year ago in the journal Animal Behaviour. Not surprisingly, military funding for the research ended long before they had obtained any evidence that crows can recognize human faces, and it is unlikely that the bird played any part in finding bin Laden.  

"So, they have a long term memory, very acute discrimination abilities, and if a group of crows knew bin Laden as an enemy, they would certainly indicate his presence when they next saw him," he says. "One of the experimental branches of research that was used to try to find him was to have crows or ravens of the local area trained to identify his face."

Related:

• Avian intelligence
• Those clever corvids

Marzluff, J., et al. (2010). Lasting recognition of threatening people by wild American crows. Anim. Behav. 79: 699-707. DOI: 10.1016/j.anbehav.2009.12.022