Things that birds know: why do pigeons bob their heads?

Omar asks: Why do pigeons shake their heads back and forth while walking

From the wording of the question it is clear that you have fallen victim to an optical illusion: the pigeons do not move their heads backwards (in relation to the environment) but only forwards. The strange head-bobbing of the pigeons is shared by many species of birds, including our friend the chicken. In particular, this ridiculous movement is common when the bird is looking for food on the ground. Already at the beginning of the twentieth century, an innovative invention was used: a movie camera to prove that the bird never moves its head backwards. The head remains fixed in space while the body below it continues to move forward. To do this the neck must bend and indeed this head swing is particularly impressive in the long-necked among the winged: ostriches, herons and cranes As you will see, for example, here (Focusing the gaze on the head and the background will convince the viewer that in relation to the external environment there is no backward head movement).

Why leave the head behind and force him to complete every step or two the gap with a sudden and funny jump? About 40 years ago, researcher BJ Frost showed that the answer lies in the sense of sight. Pigeons walking in place, on the treadmill in the gym that Frost built for them, did not shake their heads. The same thing happened to the pigeons that the visual environment that was projected to them remained unchanged while walking and no bird nods its head with its eyes covered. In the opposite experiment, pigeons were moved standing on their heads as if walking when the environment "moved" in front of them.

In a strange and unplanned way, the tendency to keep the head still was demonstrated when the treadmill that Yona was standing on moved very slowly forward (too slowly to stimulate the pigeon to walk) and the pigeon slowly stretched her neck forward until she lost her balance. We, humans and the rest of our fellow mammals, live in a movie that the brain edits for us from the transmissions transmitted to it from the eye that moves with the body. A complex system of muscles surrounds the eyeball and rotates it when we walk or run so that we can continue to focus on the important items as the field of view changes. The evolution of vision in birds developed along a different path. Anyone who has built and flown a kite knows that the nose should be light weight. The adaptation of birds to flight necessitated the reduction of the mass of the head, therefore there is no equivalent in birds to the muscle system that rotates our eye. Since birds have a very limited ability to rotate the eyeball, turning the head means losing the previous field of vision, for all the reasons that have already been identified. A bird walking on the ground, accordingly strives to keep the eye in its place as much as possible. In birds, the number of neck vertebrae can reach 14 compared to 7 in mammals, and its flexibility is such that the chicken can maintain a fixed position of the head even when the rest of its body parts are moving or rotating, The flexibility of the neck is particularly impressive in the owl Able to turn his head 270°. Thus, the pigeon sees a large part of the time a static picture of the world in which it is easy to find food on the ground. The fast forward movement may also have advantages; There were those who thought that the head of a walking pigeon reaches a speed of several tens of kilometers per hour as it leaps forward to reach the body. In this way, the pigeon moving slowly around the roundabout can take advantage of the neural mechanism that processes the image during flight and is adapted to such a range of speeds.

Using a high-speed camera, Thomas W. Cronin examined the head movements of the American whooping crane, the longest-necked of his country's birds. The eyes of the American crane are about one and a half meters above the ground, in order to capture a frog or a mouse, it must precisely land its beak down from this height, therefore focusing the sight is of critical importance. Cronin found that rocking is a well-coordinated action with walking. While one leg is planted on the ground, the movement of the body is completely horizontal and then the head is fixed in place. When the leg lands on the ground and the opposite leg rises, movement is added in the vertical axis (up and down) to the bird's body. This period of time in which it is already more difficult to achieve stability than using the head to leap forward, at this stage a refocusing of the eye to the next static image occurs. As the walking speed increases, the amount of time the head can remain stable shortens until the swaying stops completely when the crane is running. This is the limitation dictated by a long neck: the path the head has to take to line up with the body is long and with it the speed at which the head is forced to jerk forward. Birds with short necks allow themselves to continue bobbing their heads even when running.

Shaking the head is not the only mechanism by which a chicken uses the flexibility of the neck and the stability of the eyeball in its place. Owls turn their heads and shake it While their body remains at rest (opposite of the behavior of the pigeon) so that the brain receives images of the objects around it from several angles and thus an accurate assessment of distance and direction is possible just as human surveyors and mapmakers use triangulation (viewing one point from several directions). This mode of visual focus is unique to birds but similar movements have been observed in long-necked lizards and snakes. Since the head of dinosaurs was also separated from the body by a large number of vertebrae and due to the evolutionary closeness to birds, it is possible that the inhabitants of Jurassic Park also bobbed their heads just like the pigeons of today.

Thanks to Dr. Thomas Cronin for his help.

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