jones_supa writes: New Caledonian crows — already known to be smart — may also understand how to displace water to receive a reward, with the causal understanding level of a 5-7 year-old child, according to results published in the open access journal PLOS ONE by Sarah Jelbert from University of Auckland and colleagues. As demonstrated in the included video, 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. Crows completed 4 of 6 water displacement tasks, including preferentially dropping stones into a water-filled tube instead of a sand-filled tube, dropping sinking objects rather than floating objects, using solid objects rather than hollow objects, and dropping objects into a tube with a high water level rather than a low one. However, they failed two more challenging tasks, one that required understanding of the width of the tube, and one that required understanding of counterintuitive cues for a U-shaped displacement task. The authors note that these tasks did not test insightful problem solving, but were directed at the birds' understanding of volume displacement.