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.