Parrot does Maths

During one experiment Alex the parrot was presented with blocks in differently colored sets of two, three, and six. When researchers asked Alex which color group had five blocks, he answered, "None." This prompted Pepperberg to set up a series of tests in which the parrot consistently identified zero quantities of objects with the label "none."
Alex had been taught the term "none" to indicate when neither of two identically sized objects was larger than the other. He had also used it to indicate when there was no difference in other qualities, such as color or shape, among a set of objects.