From MSNBC:
A parrot has grasped the concept of zero, something humans can’t do until at least the toddler phase, researchers say. Alex, a 28-year-old African gray parrot who lives in a lab at Brandeis University in Massachusetts, has a brain the size of a walnut. But when confronted with no items on a tray where usually there are some, he says “none.” Zero is thought to be a rather abstract concept even for people. Children typically don’t grasp it until age 3 or 4, Brandeis researchers say. Some ancient cultures lacked a formal term for zilch, even as recently as the Middle Ages.
More here.