Head-bobbing in pigeons: How stable is the hold phase?

The head movement of a walking pigeon Columba livia is characterized by two alternating phases, a thrust phase and a hold phase. While the head is rap idly thrust forward during the thrust phase, it has been shown repeatedly t hat it remains virtually motionless with respect to translation along a hor izontal axis (roll axis) during the hold phase. It has been shown that the stabilization during the hold phase is under visual control, This has led t o the view that the pigeon's head-bobbing is an optokinetic response to sta bilize the retinal image during the hold phase. However, it has never been shown explicitly that the head is really held stable in space with respect to other translatory or rotatory dimensions. Using videography, we show her e that this is in fact the case: except for a small but systematic slip tha t presumably serves as an error signal for retinal image stabilization, the head of the pigeon remains locked in space not only with respect to the ho rizontal (roll) axis but also with respect to vertical translation (along t he yaw axis) and with respect to rotation around the pitch and yaw axes.