Pepperberg, I. M. (2017). Review of studies on visual perception in grey parrots (Psittacus erithacus): The Müller-Lyer illusion, amodal and modal completion. Animal Behavior and Cognition, 4(3), 378–395. https://doi.org/10.26451/abc.04.03.14.2017

Few avian studies on optical illusions are directly comparable to those with humans. Grey parrots that have some referential use of English speech, however, allow for such comparative studies, as these birds can be tested just as are humans, by asking them to describe exactly what they have seen. Here I review two studies, one on the Müller-Lyer illusion (Pepperberg, Vicinay, & Cavanagh, 2008), one on amodal and modal perception (Pepperberg & Nakayama, 2016), that demonstrate similarities between human and Grey parrot perceptual abilities.