Crows drop stones into a tube of water to raise a floating treat within reach
Researchers presented New Caledonian crows with a tube of water too low for them to reach a floating worm, along with objects to drop in. The crows chose solid, sinking objects over hollow or floating ones, and dropped them into the narrower tube with the higher water level rather than a wider one — suggesting at least some grasp of cause and effect, not just trial and error.