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In Victorian times, mammals were placed on the highest branches of Darwin’s “Tree of Life”. The famously wise old crow was knocked off its privil­eged perch, and stories of clever birds were downgraded to mere folklore. In fortean terms, they were damned data which science ignored. Birds have small brains and crucially lack the prefrontal cortex associated with intelli­gence in mammals, and that appeared to be the end of it.

Tales of crows acting intelli­gently go back at least as far as Aesop’s fable of the crow and the pitcher. Finding water level in the pitcher is too low for it to reach, the thirsty crow starts dropping in pebbles. Eventually these raise the level of the water so it can drink. It’s a charming fable about the virtues of persistence, but not a challenge to science.

There are plenty of modern tales of clever crows, like the Japanese crows that found a new way of cracking walnuts. They wait until the traffic lights turn red, and then place the nuts in the path of oncoming traffic. However, even unintelligent birds can break things open. Thrushes will seek out a stone as an ‘anvil’ to break a snail shell, and Egyptian vultures use stones to break open ostrich eggs. But more scientific research has suggested that crows are far more intelligent than had been assumed.

One strand of research has investigated tool use. There is a distinction here between simply using something found in the environment, such as the stone used by a thrush, and deliberately making a tool. The crows of New Caledonia in the South Pacific had long been reputed to make tools, but their skill was only formally described in 1996 by New Zealand biologist Gavin Hunt.

The New Caledonian crows use three different types of tool: straight twigs, hooked twigs and thorns, and tools made from Pandamus leaf. The manufacture process can be quite intricate, involving a series of cuts and tears. What makes this different from the instinctive behaviour of, say, termites, which also engage in complex construction, is that the crow can intelligently select and modify its tool-making behaviour to fit the situation.

For example, when presented the challenge of accessing food which could only be had by poking a stick through a small hole, the crows set about making a stick of the right size by stripp­ing and separating twigs from a leafy branch to reduce it to the appropriate diameter. This sort of planning and foresight can only be matched by primates. In fact, the crows did better than chimpanzees in learning to bend a piece of wire to get at food. New Caledonian crows can also use “meta-tools” (tools to alter tools), and can use up to three tools in sequence to achieve a goal.

Tool-using cleverness is one thing, but it’s the social aspect of intelligence that really sets crows apart. They learn from each other, and different patterns of tool on different islands of New Caledonia have been described as being effectively different cultures.

Crows also seem to have an understanding of each other’s behaviour and motivations. This was most notably proved in a study of scrub jays, birds that hide food in caches – and, given half a chance, steal from each other’s caches. If a jay found it was being observed when it hid food, it would go back later and hide it somewhere else.

This re-hiding behaviour is quite subtle. The jay makes sure it is out of view of a competitor; tests show that they head for shadowy spots to cache when they are observed, but not when they are on their own. And the behaviour is not instinctive; birds that have not stolen from others’ caches do not re-hide their own. It seems that only thieves are afraid of other thieves. Further, crows excel at co-operative tasks where two birds have to work together.

This gives animal psychologists something of a challenge. It is easy to interpret the jays’ behaviour in human terms; what is harder is to work out how else to explain it without invoking a social intelligence which was thought to be limited to primates.

Meanwhile, there has been a certain amount of revision on the anatomical side, which says that birds cannot possess the intelli­gence of mammals. While birds do not possess the prefrontal cortex which is connected with mammal intelligence, a part of the avian forebrain known as the nidopallium caudolaterale is anatomically and neurochemically similar. It may be significant that crows have a larger forebrain than any other group of birds except parrots (another breed with markedly high IQ). We do not yet know whether birds possess “mirror neurons” which are triggered by observing another individual performing a familiar act, and which are involved in higher cognitive functions – but if they do not, they may well possess a direct equivalent.

The final revenge of the damned data came last year when Cambridge researchers (led by one Chris Bird) repeated a copy of Aesop’s challenge in the laboratory. And two out of four rooks solved the problem on their first attempt. The fact that it took a couple of thousand years for anyone to try the experiment may say something about human intelligence.