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It's often been said crows show a remarkably high level of intelligence - they can remember faces, use tool, and communicate in sophisticated ways.

But a new study has found they are even smarter than first thought, as they can solve complex tasks previously thought possible only by humans, apes and monkeys.

And most impressively, they performed the tasks spontaneously - without any prior training.

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Research led by Lomonosov Moscow State University in Russia tested the intelligence of crows. The study found they could solve a shape-association task (shown). It involved picking similar - but not matching - shapes to find food. Two crows were found to be successful 78 per cent of the time

The research, published in Current Biology, was carried out by psychology professor Dr Ed Wasserman from the University of Iowa and three researchers from Lomonosov Moscow State University in Russia.

'What the crows have done is a phenomenal feat,' said Dr Wasserman

'That's the marvel of the results. It's been done before with apes and monkeys, but now we're dealing with a bird; but not just any bird, a bird with a brain as special to birds as the brain of an apes is special to mammals.'

The study involved two hooded crows that were at least two years old.

The birds were placed into a wire mesh cage into which a plastic tray containing three small cups was occasionally inserted.

The sample cup in the middle was covered with a small card on which a colour, shape or number of items was pictured.

The other two cups were also covered with cards - one that matched the sample, and one that didn't.

During this initial training period, the cup with the matching card contained two mealworms; the crows were rewarded with these food items when they chose the matching card, but they received no food when they chose the other card.

Once the crows had been trained on identity matching-to-sample, the researchers moved to the second phase of the experiment. 

This time, the birds were assessed with relational matching pairs of items.

These relational matching trials were arranged in such a way that the shapes on the test cards did not precisely matched the sample card, eliminating control by physical identity.

For example, the crows might have to choose two same-sized circles rather than two different-sized circles when the sample card displayed two same-sized squares.

Crows show intelligence levels only seen in humans and apes

'What the crows have done is a phenomenal feat,' said Dr Wasserman. 'That's the marvel of the results. It's been done before with apes and monkeys, but now we're dealing with a bird; but not just any bird, a bird with a brain as special to birds as the brain of an apes is special to mammals'

What surprised the researchers was not only that the crows could correctly perform the relational matches, with a success rate of 78 per cent - 50 per cent being regarded as chance - but that they did so spontaneously, without explicit training.

'That is the crux of the discovery,' Dr Wasserman said.

Russian researcher Dr Anna Smirnova studies a crow making the correct selection during a relational matching trial

'Even if brute force methods had to be employed - by giving the crowns reward on correct trials and withholding reward on incorrect trials - successful learning and transfer of relational matching behavior would have been noteworthy.'

'The fact that the crows spontaneously engaged in correct relational matching makes the present results truly remarkable.'

He added: 'This was a very artful experiment.

'I was just bowled over by how innovative it was.'

However, the researchers conceded that the earlier identity matching-to-sample training may have given them some background knowledge for this more complex test.

Nonetheless, even that in itself would be an 'intriguing matter for future study,' they wrote in their paper.

Dr Anthony Wright, neurobiology and anatomy professor at the University of Texas-Houston Medical School, said the discovery ranks on par with demonstrations of tool use by some birds, including crows.

'Analogical reasoning, matching relations to relations, has been considered to be among the more so-called "higher order" abstract reasoning processes,' he said.

'For decades such reasoning has been thought to be limited to humans and some great apes. The apparent spontaneity of this finding makes it all the more remarkable.'

Dr Joel Fagot, director of research at the University of Aix-Marseille in France, agreed the results shatter the notion that 'sophisticated forms of cognition can only be found in our "smart" human species. Accumulated evidence suggests that animals can do more than expected.'

Dr Wasserman conceded there will be skeptics and hopes the experiment will be repeated with more crows as well as other species. He suspects researchers will have more such surprises in store for science.

'We have always sold animals short,' he said. 'That human arrogance still permeates contemporary cognitive science.'