Investigations into the Biomechanics of Head-Bobbing Pigeons
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Walking pigeons display a characteristic behaviour: head-bobbing. Head-bobbing consists of two phases, the hold phase during which the head is kept stable in space, and the thrust phase during which the head is quickly moved forward. But why do pigeons and so many other birds head-bob? Two theories have been developed to answer this question. First, head-bobbing may have a visual function, namely object detection during the hold phase and depth perception during the thrust phase. Second, head-bobbing may have the biomechanical function of increasing the stability. The present study aimed to gain further insight into why pigeons head-bob. A gait cycle analysis of walking pigeons revealed that the hold phase starts at a specific point in the gait cycle and is not dependent on the walking speed. The point in the gait cycle at which the hold phase starts indicates a potential increase in stability during the single stance phase, as it leads to a longer period of time during which the center of mass is above the base of support. The end of the hold phase, however, is dependent on the walking speed, specifically the step length for a given walking speed. I found that the thrust phase duration is constant for all walking speeds and the distance the head travels during that time is equal to the step length. With increasing walking speed the head moves faster during the thrust phase and the hold phase duration is decreased leading to a higher head-bobbing frequency. With fast enough walking speed the hold phase is eventually eliminated, marking the transition point from walking with head-bobbing to fast walking without head-bobbing. The exact velocity of this transition point was predicted for each individual and was dependent on the maximal step length of each pigeon. All head-bobbing parameters could be described with the step-frequency, the thrust phase duration, and the maximal step length, indicating that head-bobbing behavior is individual for every pigeon and depends on its body size.