Head-Bobbing During Walking, Running and Flying: Relative Motion Perception in the Pigeon

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Abstract
Measurements of head and body velocity were made by single-frame analysis of films showing pigeons walking, running, flying towards a perch and taking off from the ground. A rhythm of head velocity was always observed in the first three situations, but never in the last. During walking, this head-bobbing rhythm resulted in stabilization of the head relative to the surroundings for a part of each cycle. During running or flying towards a perch, this stabilization did not occur, and the head continued to move forwards throughout the cycle. The velocity of head movement at the minimum of the cycle increased smoothly with body speed in the range of body speeds from 75cms−1 upwards, with no indication of any discontinuity between running and flight. The results provide evidence that headbobbing is not a mechanical consequence of cursorial or flight activity, and that its visual functions must include others besides stabilization of the retinal image. It is suggested that head-bobbing has the dual function of amplifying relative motion in the retinal image during the thrust phase, making the detection of food objects more likely, while allowing detection of object motion during the hold phase.