Several bird species move their heads back and forth during walking. This head-bobbing is one of the most striking features of avian locomotion. It has been two functional implications about this movement: visual and postural stabilization. This study examined whether head bobbing in chickens (Gallus gallus domesticus) is controlled by walking itself or visual motion accompanied with walking using treadmill apparatus.

Six chickens were trained to walk on a treadmill. Two LCD monitors were arranged on both sides of the treadmill. While the chicken walked with the various treadmill speed, random dots moving to the same direction as the treadmill were presented on the lateral monitors. The chicken’s head position was recorded from above the apparatus using a video camera and traced by originally developed object tracking system. We found that (1) backward head movements occurred with presentation of backward motion stimulus, (2) no consistent movements occurred with presentation of static stimulus, (3) the velocity of head movements was significantly correlated with the velocity of dots, and (4) walking speed had little effect on velocity or frequency of head movements. These results indicate that head-bobbing in chickens is tuned to visual motion velocity, not walking speed. Further experiment revealed that the head movements were affected by frontal expanding stimuli, suggesting that head-bobbing is elicited by global motions or optical flow, not by local motions.

The present study suggests that visual stabilization is the dominant function of head-bobbing and postural stabilization is secondarily adapted. This head movements is analogous to optokinetic response with eyes in humans and other mammals. Comparing head and eye stabilization should reveal their phylogenetic origins and explain differences of required visual cognitive ability in each species.