Budgerigars (Melopsittacus undulatus) perceive the Müller-Lyer illusion.

A Müller-Lyer figure consists only of a line and arrowheads located at both ends of the line. Many comparative studies have reported that animals perceive Müller-Lyer illusion as humans, but few have used appropriate experimental designs to verify whether animal subjects actually respond to line length alone. The present study investigated whether budgerigars (Melopsittacus undulatus) can perceive the Müller-Lyer illusion by using a method that addresses this problem. Four budgerigars were trained to select a long or short line (counterbalanced across subjects) from two horizontal lines. Next, the same task was conducted using two lines, one of which was situated between arrowheads pointing either right (>>) or left (<<). In the final training phase, the arrowheads were replaced with those pointing inward (><) or outward (<>). The performance of each subject toward each stimulus set of these trainings suggested that they did not determine the length of the line by including the arrowheads. In the test phase, response tendencies to the four figures were compared. Results suggested that budgerigars perceive the Müller-Lyer illusion in the same direction as humans; however, its magnitude is larger than that of humans. (PsycInfo Database Record (c) 2022 APA, all rights reserved).