According to researchers at Brandeis University in Massachusetts, a bird with a walnut-size brain has grasped the concept of zero, something humans don't usually understand until age three or four. When asked about the contents of an empty tray, Alex, a 28-year-old African gray parrot, correctly answers "none." A remarkable achievement given some ancient human cultures did not even have a formal concept of zero. Read more at LiveScience(1).

1. http://www.livescience.com/animalworld/050708_parrot_zero.html