"We show that hand-raised juvenile New Caledonian crows spontaneously manufacture and use tools, without any contact with adults of their species or any prior demonstration by humans," said Alex Kacelnick, of Oxford University in England.

The crows -- three males and one female -- were raised in artificial nests and then transferred to aviaries that contained a variety of twigs and food hidden in crevices.

Two of the birds were shown by their human foster parents how to pry food out of tiny spaces with the twigs but the other birds were not.

"All four crows developed the ability to use twig tools," Kacelnick and his colleagues said in a report in the science journal Nature.

The tutored birds watched the tool demonstrations but the scientists found no difference in their skills and those of the other two birds.

"In light of our findings, it is possible that the high level of skill observed in wild adult crows is not socially acquired," said Kacelnick.