New Zealand scientists have helped show that some crows are as clever as five-to-seven-year-old children in some experiments.
Scientists from Auckland University and Britain's Cambridge University assessed the brainpower of wild New Caledonian crows in six experiments.
Through training, the birds learnt to figure out the best stones to drop into water-filled tubes in order to raise a floating piece of meat high enough to grab. They rejected sand-filled tubes and lower water levels.
Crows commonly use twigs as tools but are not known to drop stones into containers or water in the wild.
The results have been published in an American Public Library of Science journal.