Crows are smart. In 2014, University of Auckland psychologist Sarah Jelbert and her colleagues assessed the causal understanding of water displacement in New Caledonian crows by presenting them with a narrow tube in which a reward floated out of reach. To get the reward, a bird had to drop objects into the tube to raise the water level.
“We found that crows preferentially dropped stones into a water-filled tube instead of a sand-filled tube; they dropped sinking objects rather than floating objects; solid objects rather than hollow objects, and they dropped objects into a tube with a high water level rather than a low one.”
(Sarah A. Jelbert et al., “Using the Aesop’s Fable Paradigm to Investigate Causal Understanding of Water Displacement by New Caledonian Crows,” PloS One 9:3 , e92895.)