New Caledonian crows are as good at reasoning as a human seven-year-old, claim researchers.

The scientists subjected six wild crows to a battery of tests designed to challenge their understanding of causal relationships. The "water displacement" tasks were all variations of an Aesop's fable in which a thirsty crow drops stones to raise the level of water in a pitcher.

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

They demonstrated an ability to choose objects that would sink rather than float, and solid rather than hollow objects, and they were able to choose a high water level tube over one with a low water level, and a water-filled tube over one filled with sand.

The crows failed on two more difficult tasks. One required an understanding of the width of the tube and the other involved displacing water in a U-shaped tube.

Nevertheless, the birds' understanding of the effects of volume displacement matched that of human children aged five to seven, said the scientists.

Lead researcher Sarah Jelbert from the University of Auckland, New Zealand, said: "These results are striking as they highlight both the strengths and limits of the crows' understanding.

"In particular, the crows all failed a task which violated normal causal rules, but they could pass the other tasks, which suggests they were using some level of causal understanding when they were successful."

New Caledonian crows, named after the Pacific islands where they live, are famous for their intelligence and inventiveness. They are the only non-primate species known to fashion tools, such as sticks and hooks which they use to winkle out grubs from logs and branches.

The findings appear in the latest issue of the online journal PLOS ONE.