Crows are as clever as your seven-year-old!

Aesop's fable that told us how smart the thirsty crow was. Scientists have now discovered that crows are as intelligent as a seven-year-old child.

Their abilities to make tools, such as prodding sticks and hooks which they use to pick up out grubs from logs and branches, have been counted for their smartness.

Scientists experimented with six wild New Caledonian crows to challenge their understanding of cause and effect.

The tasks were all variations of the fable in which a thirsty crow drops stones to raise the level of water in a pitcher.

In the water displacement task, crows worked out how to catch floating food rewards by dropping heavy objects into water-filled tubes.

The crows demonstrated an ability to drop sinking rather than floating objects, solid rather than hollow objects, to choose a high-water level tube over one with low-water level, and a water-filled tube over one filled with sand, the researchers said.

"The birds' understanding of the effects of volume displacement matched that of human children aged between five and seven," said Sarah Jelbert from University of Auckland, New Zealand.

These results are striking as they highlight both the strengths and limits of the crows' understanding, said the study published in the journal PLOS ONE.