New Caledonian crows may understand how to displace water to receive a reward, with the causal understanding level of a 5-7 year-old child.
Scientists used the Aesop's fable riddle— in which subjects drop stones into water to raise the water level and obtain an out-of reach-reward—to assess New Caledonian crows' causal understanding of water displacement. These crows are known for their intelligence and innovation, as they are the only non-primate species able to make tools, such as prodding sticks and hooks.
Six wild crows were tested after a brief training period for six experiments, during which the authors noted rapid learning (although not all the crows completed every experiment). The authors note that these tasks did not test insightful problem solving, but were directed at the birds' understanding of volume displacement.