Motion parallax via head movements modulates visuo-motor control in pigeons.

Although it has been proposed that birds acquire visual depth cues through dynamic head movements, behavioral evidence is lacking about how birds use motion parallax depth cues caused by self-motion. This study investigated whether self-generated motion parallax modulates pecking motor control and visual size perception in pigeons (Columba livia). We trained pigeons to peck a target on a touch monitor and to classify it as small or large. To manipulate motion parallax of the target, we changed the target position on the monitor according to the bird9s head position in real time using a custom-built head tracker with two cameras. Pecking motor control was affected by the manipulation of motion parallax: when the motion-parallax signified the target position farther than the monitor surface, the head position just before pecking to target was near to the monitor surface, and vice versa. By contrast, motion parallax did not affect how the pigeons classified target sizes, implying that motion parallax might not contribute to size constancy in pigeons. These results indicate that motion parallax via head movements modulates pecking motor control in pigeons, suggesting that head movements of pigeons have the visual function of accessing motion parallax depth cues.