New Zealand scientists have helped show that some crows are as clever as depicted by the storyteller Aesop in the ancient Greek fable, The Crow and the Pitcher.
Scientists from Auckland University and the UK's Cambridge University say the birds' brainpower - at least in this one area - rivals that of five-to-seven-year-old children.
To assess their causal understanding of water displacement, six wild New Caledonian crows (Corvus moneduloides) were subjected to six experiments.
They were similar to the fable, in which a thirsty crow drops stones into a pitcher containing a little water until it rises high enough for him to drink.
Although the birds commonly use twigs as tools, they are not known to drop stones onto containers or into water in the wild.
So before the experiments began, they were trained to drop stones into a tube.
In the experiments, the crows had to drop objects into water-filled tubes to raise the water to a level enabling them to reach a floating cork with a cube of attached meat.
They preferentially dropped stones into a water-filled tube instead of a sand-filled one; and they dropped sinking objects rather than floating ones, solid objects rather than hollow ones, and objects into a tube with a high water level rather than a low one.
However, they failed two more challenging tasks.
In one, they had to choose between a narrow and a wide tube with equal water levels.
In the other, which was counterintuitive, they faced three tubes with equal levels of water, the middle one baited but too narrow to drop stones into, and having a concealed connection to one of the outer tubes beneath an opaque base.
The results of the study, published in the US-based scientific journal PLOS ONE, suggest that "New Caledonian crows do possess a causal understanding of displacement, but this understanding has limits," the scientists write.
They described the birds' ability as "striking."
