In 2005, researchers reported that a grey parrot named Alex used the word 'none' for an absent quantity, suggesting a zero-like concept

A 2005 study revealed an African grey parrot named Alex understood an empty set.

Alex could apply the label 'none' to absent object categories.

This parrot had previously learned to identify objects, colours, and quantities.

Researchers interpreted these findings as evidence of a zero-like concept.

The experiment suggested some cognitive processes for representing absence are not unique to humans.