The New Caledonian crow has surprised English researchers with its tool-making skills.

The birds are already known for their tool-making behaviour in the wild. However, Professor Alex Kacelnik and his colleagues at the University of Oxford have observed a new approach that probably would not work with natural materials.

They report in this week's Science an experiment inspired by their captive female crow, Betty, who spontaneously bent a piece of straight wire into a hook then successfully used it to lift a bucket of food from a vertical pipe.

This occurred during the fifth trial of another experiment in which Betty and her male companion, Abel, had to choose between a hooked and a straight wire to reach some food.

The paper explains how on one occasion, Abel stole the hook. But instead of giving up, Betty proceeded to bend the unusable straight wire into a hook all by herself, to obtain the food.

"She used brains rather than muscle," said Professor Kacelnik.

To investigate the importance of this observation, the researchers designed a new test where they placed a single straight piece of garden wire on top of the food tube to see what happened.

During 10 trials, Betty bent the wire and used it to retrieve the food nine times. Abel retrieved the food once with the straight wire.

In the wild, New Caledonian crows take twigs with branchings, which they remove until they have a hook.

"Bending a twig won't work because twigs either break or don't keep their new shape," said Professor Kacelnik.

While a few animals, mostly primates, are known to use existing objects as tools, the purposeful modification of objects by animals for use as tools, without extensive prior experience, is virtually unknown.

Certainly no animal has ever been observed bending a piece of wire to make a hook.

"Indeed, these observations show a level of competence and understanding of the function of hooks unknown as yet outside our own species," said Professor Kacelnik.

These findings suggest numerous intriguing topics for further study, such as tracing the underlying neural and cognitive mechanisms that make these birds - rather than our primate relatives - our closest kin when it comes to making tools.

"We also don't know yet to what other aspects of intelligence this may extend," Professor Kacelnik said.