What abilities did Alex the African gray parrot demonstrate with English words?

The African gray parrot named Alex, studied by psychologist Irene Pepperberg, demonstrated that parrots could associate sound with meaning, moving beyond simple mimicry.1

His abilities with the English language included:

• Counting: He could count up to six.1
• Categorization: He could correctly label objects based on five shapes and seven colours.1
• Differentiation: He could distinguish groups of objects by their material, shape, and colour.1
• Social Communication: He used English to interact with other African grays in the laboratory, where he would encourage or chastise their attempts at labeling and categorization.1

These findings were significant because they provided evidence that parrots possess a level of comprehension and cognitive ability comparable in some respects to that of a three- or four-year-old human child.12