Vowel triangle area is a phonetic measure of the clarity of vowel articulation. Compared with speech to adults, people hyperarticulate vowels in speech to infants and foreigners but not to pets, despite other similarities in infant- and pet-directed-speech. This suggests that vowel hyperarticulation has a didactic function positively related to the actual, or even the expected, degree of linguistic competence of the audience. Parrots have some degree of linguistic competence yet no studies have examined vowel hyper- articulation in speech to parrots. Here, we compared the speech of 11 adults to another adult, a dog, a parrot, and an infant. A significant linear increase in vowel triangle area was found across the four conditions, showing that the degree of vowel hyperarticulation increased from adult- and dog-directed speech to parrot-directed speech, then to infant-directed speech. This suggests that the degree of vowel hyperarticulation is related to the audience's actual or expected linguistic competence. The results are discussed in terms of the relative roles of speakers' expectations versus listeners' feedback in the production of vowel hyperarticulation; and suggestions for further studies, manipulating speaker expectation and listener feedback, are provided.