Jennifer Ackerman, The Genius of Birds (New York: Penguin Books, 2016), p. 86:
Whether New Caledonian crows have leaps of insight remains to be determined, but these experiments suggest that these birds do have an extraordinary ability to notice the consequences of their own actions, says Taylor, and to pay attention to the way objects interact. These are mighty useful mental tools when it comes to making and using material tools. THE AUCKLAND TEAM IS also attempting to figure out whether the crows understand basic physical principles.A “crow-appropriate paradigm” for this, as Taylor puts it, is an experimental version of the old Aesop’s fable “The Crow and the Pitcher.” In that fable, a thirsty crow comes across a half-filled jug of water. Unable to reach the water to drink, the crow drops pebble after pebble into the pitcher until the water level rises enough for him to drink. As it turns out, this is not just a folktale. New Caledonian crows will do exactly that—drop stones into a water-filled tube to raise the water level. And, as Sarah Jelbert discovered while working with the Auckland team, if given a choice between heavy objects and light ones, solid and hollow ones, the crows will spontaneously pick objects that will sink over those that will float. They know how to pick their materials and will select the right option 90 percent of the time. This suggests that the crows understand water displacement, a fairly sophisticated physical concept, on par with the comprehension of a child five to seven years old. It also suggests that they’re able to grasp the basic physical properties of objects and make inferences about them.