Flickr/Leonard John Matthews

Crows can recognise and ascribe numerical meaning to symbols, a new study shows, suggesting that the unusually intelligent birds may be able to “read” numbers and simple icons.

Crows are known for their extraordinarily good memories, tool-making skills and ability to discern minute subtleties in judging a threat level. It was reported earlier this year that the US military considered using crows to help track down al-Qaeda leader Osama bin Laden.

In the latest edition of the journal Animal Behaviour, Japanese researchers describe an experiment in which eight jungle crows were presented with two containers, one with “2” written on the lid and one with “5”. The “5” container had food inside, while the “2” did not. The crows soon learned to pick the “5” container at a 70% success rate.

Other experiments tested whether the crows could differentiate between containers marked with non-numerical symbols such as shapes. The birds scored a 70 to 90% success rate picking the food-filled container for 19 out of 20 non-numerical symbol tests.

“These results suggest that jungle crows have a natural tendency to select the larger quantities and that decisions were affected by the numerical ratio and stimuli magnitude, indicating the use of analogue magnitude mechanism for numerical judgement,” the researchers said in their paper.

Dr Stephen Debus, a bird expert and honorary research associate in zoology at the University of New England, said the results were interesting but not surprising because crows were renowned for their superior intelligence.

It is unclear why the birds evolved such smarts, he said “but I gather that it is probably related to their complex social organisation and also, being omnivorous in complex environments, they need to be able to find food in novel situations and solve problems in obtaining that food.”

He said he expected the study of crows to reveal more of their skills in future.

“I’ve heard it said that crows can count, and that the usual bird photographer’s ruse of having a ‘seeing-in’ party accompany the photographer to a hide, then depart (leaving the photographer in the hide to photograph the unsuspecting bird), doesn’t work with crows, because they see how many people went into the hide and how many then left. That is, they know that someone is still in the hide and so won’t approach within ‘shooting’ range,” said Dr Debus, who was not involved in the Japan study.

“They seem to know the effective range of guns, too, and the difference between an armed and unarmed person, and that they’re safe and approachable in areas such as cities, where guns can’t be used.”