An African grey parrot spontaneously grasped a 'zero-like' concept

Alex, trained for decades by psychologist Irene Pepperberg, already used the word 'none' to mean no difference between two objects. In a 2005 test he spontaneously applied it to an empty tray, without ever being taught to count to zero. Outside researchers called it a genuine, if basic, zero-like concept, the first time a bird had shown one.

— Irene Pepperberg, Alex (parrot) — Journal of Comparative Psychology, 2005