Aesop's rook: The birds raise the water by dropping stones into a tube so they can reach a floating worm
One of Aesop's fables may have been based on fact, scientists report.
In the tale, written more than 2,000 years ago, a crow uses stones to raise the water level in a pitcher so it can reach the liquid to quench its thirst.
Now a study published in Current Biology reveals that rooks, a relative of crows, do just the same when presented with a similar situation.
The team says the study shows rooks are innovative tool-users, even though they do not use tools in the wild.
Another paper, published in the journal Plos One, shows that New Caledonian crows - which like rooks, are a member of the corvid group, along with ravens, jackdaws, magpies and jays - can use three tools in succession to reach a treat.
The crow and the pitcher fable was used by Aesop to illustrate that necessity is the mother of invention. But until now, the morality tale was not thought to have a grounding in fact.
Nowadays, we've had so many startling findings that the rooks just don't surprise me that much any more. You almost expect them to do the cleverest thing
Nathan Emery, QMUL
To investigate further, a team from the University of Cambridge and Queen Mary, University of London (QMUL) presented four captive rooks with a set-up analogous to the fable.
The birds were shown a clear tube containing a small amount of water. Floating upon it was an out-of-reach worm. And a pile of stones was positioned nearby.
Dr Nathan Emery, co-author of the paper, from QMUL, said: "The rooks have to put multiple stones in the tube until the worm floats to the top."
And the four birds did just that. Two, called Cook and Fry, raised the water-level enough to grab the floating feast the very first time that they were presented with the test, while Connelly and Monroe were successful on their second attempt.
Footage of the experiments shows the rooks first assessing the water level by peering at the tube from above and from the side, before picking up and dropping the stones into the water.
The birds were extremely accurate, using the exact number of stones needed to raise the worm to a height where they could reach it.
This experiment shows how the remarkable rooks opt for larger stones over smaller ones to raise the water level more quickly
In another experiment, the rooks were presented with a similar scenario. This time they were given a combination of small and large stones.
Rooks are a member of the corvid family of birds
Overall, Dr Emery told BBC News, the rooks opted for the larger ones, raising the worm to the top of the tube more quickly.
He said: "They are being as efficient as possible."
And when given a choice between a tube filled with water and another filled with sawdust, the birds were more likely to opt for the liquid-filled tube.
The researchers say their findings suggest that Aesop's ancient fable may have been based on fact.
They said: "In folklore, it is rarely possible to know with certainty which corvid is being referred to.
"Hence, Aesop's crow might have easily been Aesop's rook."
Earlier this year, the same team revealed that rooks were able to use different tools to solve a variety of complex problems.
Dr Emery told BBC News: "I used to say, maybe two or three years ago, that everything they did surprised me.
"But nowadays, we've had so many startling findings that the rooks just don't surprise me that much any more. You almost expect them to do the cleverest thing."
The only other animals reported to have solved an Aesop-like problem are orangutans.
Christopher Bird, co-author on the paper, added: "Corvids are remarkably intelligent, and in many ways rival the great apes in their physical intelligence and ability to solve problems."
This New Caledonian crow used three tools in succession to reach a tasty treat
A different study published this week has also shed light on corvid intelligence.
A team at the University of Oxford found that New Caledonian crows were able to use three tools in succession to reach a reward.
These birds, which are found on the Pacific island of New Caledonia, use tools in the wild, crafting them from branches to pluck grubs from holes and crevices.
But this study builds on their tool-using repertoire.
Captive crows were presented with several horizontal tubes. One of the tubes contained some out-of-reach food. The others contained long and medium-length hooks - but, again, these were all out of beak's reach. And a shorter hook-like tool was positioned nearby.
The researchers found that the birds picked up the short tool, then used this to grasp the medium-length tool, which they then employed to retrieve the longest tool from the tube. Finally, they were able to use this to drag out the tasty morsel.
New Caledonian crows use tools in the wild
Four out of seven of the birds tested were able to use three tools in the right order, the team said.
Professor Alex Kacelnik, an author on the paper from Oxford's Behavioural Ecology Group, said: "The essence of our paper is to try to understand the mental processes used by the animals to actually achieve their goals."
He said that the complexity of the task made it unlikely that the crows were solving the problem using trial and error.
He added: "We are aware that the animals probably do it by putting together, in creative ways, things that they have learned individually."
Researchers believe that an ancient ancestor of the corvids might have evolved the capacity to use tools, and that all members of the corvid family may have this innate ability.
However, only New Caledonian crows draw upon it in the wild, potentially because of ecological pressures.
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