The term head-bobbing, used for the apparent back and forth movement of the head of some birds while walking on the ground (think pigeons), is a bit of a misnomer. These birds don't actually bob their heads back and forth – instead, they move their body forward, leaving the head behind, then thrust their head forward past their body (hold, high-speed thrust!). This innate behaviour is believed to help several key aspects of vision during locomotion and foraging. While the head is still, images are stabilized on the retina and moving objects become clear against the background, and during the thrust phase, depth perception becomes more accurate. Although the benefit to vision may be clear, there remains some debate as to whether head-bobs and leg movements are synchronized or coordinated during terrestrial locomotion in birds, and how head and leg movements may affect each other. In a recent paper in Ibis, Jennifer Hancock of Marietta College, USA, and colleagues Nancy Stevens and Audrone Biknevicius of Ohio University Heritage College of Osteopathic Medicine, USA, studied heartily head-bobbing, rapidly running elegant-crested tinamous in order to shed light on this kinematic conundrum.
To test whether head-bobbing and leg movements during terrestrial locomotion were synchronized or coordinated in birds, Hancock and her colleagues filmed three elegant-crested tinamous as they moved at a range of speeds across a track, and recorded the forces of each foot movement. They then analyzed the timing of the beginning of each type of event (head hold and thrust phases, and touchdown and liftoff of each foot) to determine the relationship between head and leg movements. They also estimated the pitch of the body during walking and running, to determine whether head-bobbing tilted the bird's body forward or backward during locomotion.
Hancock and her colleagues found that as the tinamous ran faster, the hold phase of the head-bob did not occur. Essentially, they ran with their head thrust forward, a result also seen in pigeons. The most striking finding, however, was that head-bobbing and leg movements were not well coordinated in most of the trials they performed. Interestingly, although the head and leg movements of Tinamous were not coordinated in this study, head-bobbing did appear to affect locomotion. When tinamous walked slowly, the steps that coincided with a hold phase of the head were held for longer – to help smooth out that stride while images were being stabilized on the retina. Lastly, they found that the body pitched forward, again during the critical hold phase, to help stabilize the head.
This study showed no evidence of synchronicity between head-bobbing and leg movements during locomotion in tinamous, a finding that contradicts an often-assumed theory that these two events are synchronized. However, what Hancock and her colleagues did find was that locomotion appears to be constrained by head-bobbing, or rather by the visual constraints that trigger head-bobbing. Although this study only examined the relationship between head-bobbing and locomotion in tinamous, it sheds new light on an old theory. Perhaps there is more to head-bobbing than once thought, and maybe it needs a new name.