Budgerigars (Melopsittacus undulatus) perceive the Müller-Lyer illusion

Area of Science:

• Comparative psychology
• Animal cognition
• Visual perception

Background:

• The Müller-Lyer illusion is a well-known optical illusion affecting human depth perception.
• Previous studies suggest animals may perceive this illusion, but experimental designs often lack rigor in isolating line length perception.

Purpose of the Study:

• To investigate whether budgerigars (Melopsittacus undulatus) perceive the Müller-Lyer illusion.
• To employ an experimental design that ensures budgerigars respond to line length independently of arrowhead cues.

Main Methods:

• Four budgerigars were trained to discriminate line lengths.
• Stimuli included lines with inward, outward, and outward-pointing arrowheads.
• A controlled testing phase compared responses to different Müller-Lyer figure configurations.

Main Results:

• Budgerigar performance indicated they did not incorporate arrowhead length into their line length judgments.
• Results suggest budgerigars perceive the Müller-Lyer illusion in the same direction as humans.
• The magnitude of the Müller-Lyer illusion perception in budgerigars is greater than in humans.

Conclusions:

• Budgerigars demonstrate the ability to perceive the Müller-Lyer illusion.
• This study provides robust evidence for illusory perception in birds, independent of confounding visual cues.
• The findings contribute to understanding the cross-species generality of visual illusions and avian visual processing.