In Aesop's fable "The crow and the pitcher," written thousands of years ago, a thirsty crow used stones to raise the level of water in a pitcher and quench its thirst. Now, a new report published online on August 6th in Current Biology, a Cell Press publication, suggests that the fable might have been based on fact.
Rooks, intelligent birds belonging to the crow family, can indeed learn to use stones to raise the level of water, in this case to bring a tasty, floating worm within reach.
The only other animal known to complete a similar task is the orangutan, said Christopher Bird of the University of Cambridge. They have been able to bring a peanut within their grasp by spitting water into a tube, according to earlier reports.
"Corvids are remarkably intelligent, and in many ways rival the great apes in their physical intelligence and ability to solve problems," Bird said. "This is remarkable considering their brain is so different to the great apes'. Although this has been speculated in folklore, empirical tests are needed to examine the extent of their intelligence and how they solve problems."
All four rooks tested were able to complete the task set for them in the new study, using the stones to raise the water level to the height at which the worm could be reached, the researchers report. Birds named "Cook" and "Fry" were successful on their first attempt, and "Connelly" and "Monroe" took two tries. (Fry dropped out early, however, after having an adverse reaction to a wax worm.)
The birds were also highly accurate in their ability, putting in only the exact number of stones needed to raise the water level to the reachable height. Rather than trying to get the worm after each stone was dropped, they apparently estimated the number needed from the outset and waited until the time was right.
The rooks selected larger stones over smaller ones to get the most bang for their buck, so to speak. In other experiments, the birds caught on quickly to the notion that sawdust cannot be displaced in the same way that water can be.
Despite their impressive ability, rooks aren't thought to use tools in the wild at all.
"Wild tool use appears to be dependent on motivation," Bird said. "Rooks do not use tools in the wild because they do not need to, not because they can't. They have access to other food that can be acquired without using tools."
As Bird noted, that fits nicely with Aesop's maxim, demonstrated by the crow: "Necessity is the mother of invention."
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The authors include Christopher David Bird, University of Cambridge, Cambridge, UK; and Nathan John Emery, Queen Mary University of London, London, UK.