Alex, a Grey parrot (Psittacus erithacus) who identifies the bigger or smaller of two objects by reporting its color or matter using a vocal English label and who states "none'' if they do not differ in size, was presented with two-dimensional Müller-Lyer figures (Brentano form) in which the central lines were of contrasting colors. His responses to ``What color bigger/smaller?'' demonstrated that he saw the standard length illusion in the Müller-Lyer figures in 32 of 50 tests where human observers would also see the illusion and reported the reverse direction only twice. He did not report the illusion when (a) arrows on the shafts were perpendicular to the shafts or closely approached perpendicularity, (b) shafts were 6 times thicker than the arrows, or (c) after being tested with multiple exposures--conditions that also lessen or eliminate the illusion for human observers. These data suggest that parrot and human visual systems process the Müller-Lyer figure in analogous ways despite a 175-fold difference in the respective sizes of their brain volumes. The similarity in results also indicates that parrots with vocal abilities like Alex's can be reliably tested on visual illusions with paradigms similar to those used on human subjects.
Pepperberg, I. M., Vicinay, J., & Cavanagh, P. (2008). Processing of the Müller-Lyer illusion by a grey parrot (Psittacus erithacus). Perception, 37(5), 765-781.