The present study investigated how budgerigars and humans perceive a version of the Delboeuf illusion. In Experiment 1, we trained 4 budgerigars to discriminate between the sizes of two square targets, one of which was embedded in a concentric square frame. The birds accurately differentiated between the sizes of the targets when the target size disparity was large; however, when this disparity was small, they tended to choose the target embedded in the frame. Experiment 2 used the same stimuli and task as Experiment 1 but the participants were humans; the results suggested that the human participants perceived a normal Delboeuf illusion. Thus, these results indicate the adequacy of our stimuli and the inadequacy of tasks with two choices in comparative studies of the Delboeuf illusion as the simultaneous presentation of the two illusory figure(s) may cause unexpected choice bias.

Key words: Delboeuf illusion, simultaneous discrimination, budgerigars