In the story of “The crow and the Pitcher” from Aesop's Fables, a thisty crow (乌鸦)drops stones into a narrow jar to raise the low level of water inside so he can take a drink.

    Now scientists have evidence to back up that story. Crows actually do understand how to make water displacement (移位) work to their advantage, experiments show. The results suggest that the birds are, at least in some aspects, as smart as first-graders.

    Researchers, led by Sarah Jelbert at the University of Auckland in New Zealand, presented six crows with tubes filled with water. Inside the tubes, a worm or piece of meat on a piece of wood was floating, just out of reach of the crow. In front of the tubes, the researchers arranged several rubber erasers that would sink, and some plastic objects that would float. The crows found out that they could drop the erasers into the tubes in order to raise the water level and get their snack.

    However,the birds handled awkwardlyin experiments in which they could choose to drop objects in either a wide tube or a narrow one to get a snack, the researchers said. Dropping objects into narrow tube would lift the water level by a greater amount and put the treat within reach after just two drops; while it took around seven drops to raise the snack to the same level in the wide tube. The crows obviously didn't realize this, and most of them went for the wide tube first.

    Previous studies showed that chimps and human children can solve similar tasks. In a 2011 study, chimps and kids found out that they could put water into a tube to reach a peanut that was floating in a small amount of water at the bottom.