One of Aesop's fables² may have been based on fact, scientists report.

科学家发现，一则伊索寓言可能建立在事实的基础上。

Rooks are a member of the corvid family of birds

In the tale, written more than 2,000 years ago, a crow（乌鸦） uses stones to raise the water level in a pitcher³（水罐） so it can reach the liquid to quench⁴（结束，解渴） 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 innovative⁵ 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 ravens⁶（乌鸦）, jackdaws（穴鸟）, magpies⁷（喜鹊） and jays（松鸡） - can use three tools in succession to reach a treat.

Floating feast

The crow and the pitcher fable¹ was used by Aesop to illustrate⁸ that necessity is the mother of invention. But until now, the morality tale was not thought to have a grounding in fact.

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 analogous⁹ to（类似于） the fable.

The birds were shown a clear tube containing a small amount of water. Floating upon it was an out-of-reach worm（虫，蠕虫）. And a pile of stones were 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.

In another experiment, the rooks were presented with a similar scenario¹⁰. This time they were given a combination of small and large stones.

Overall, Dr Emery told BBC News, the rooks opted¹²（选择） 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（锯屑）, the birds were more likely to opt¹¹ 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 folklore¹³（民间传说）, 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."