As I strolled through Hermann Park on a sunny but chilly afternoon, I noticed a huge flock of birds flying down to the edge of the park’s lake from a tall, bare tree that they were resting in. I sat at a bench to watch these birds that seemed to stick together as they flew or pecked on the ground. Although I was watching these birds at the park, I knew that I had seen them around the city, especially at the MetroRail station, gathering at areas where they knew they could get food. These birds had plump bodies covered in light grey feathers with two black bands on their sometimes speckled wings. However, their small, round heads and necks were covered in dark grey feathers with traces of iridescent purple and green. With red eyes, they stared at their surroundings while walking on their red legs. These birds were none other than Columba livia, also known as the rock pigeon.
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Rock pigeon. Photo by Razvan Socol (CC BY-SA 3.0).
As I observed the rock pigeons, these birds behaved as they usually did whenever I saw them. They flew or walked, pecking the ground for food; however, I saw a behavior that I had never noticed before and that intrigued me: the bobbing of their heads. As the rock pigeons walked, their heads seemed to slowly move back and forth. As I chased after them, the pigeons would walk quickly with their head rapidly bobbing back and forth, most likely feeling threatened. This made me wonder, “Why do the pigeons bob their heads while walking?”
The most likely explanation behind the movement of the pigeon’s head is vision. A pigeon’s head does not actually move back and forth. Rather, the head moves in two different phases: hold and thrust phases. During the hold phase, the head stays still, but the body continues to move forward, giving the illusion that the head is moving backwards. Then the head catches up with the rest of the body during the thrust phase (Troje and Frost, 2000). The pigeon’s eyes are located on the sides of its head and are used separately, allowing for a wider area of vision. However, there is a trade-off for this monocular vision; because the eyes are located at the sides and their views do not overlap, the pigeon has a poor sense of depth perception (Catania, 1964). Pigeons or their ancestors must have acquired the behavior of bobbing their heads in order to compensate for this deficit. During the hold phase, the head is stationary, allowing for the pigeon to stabilize their vision and have a clearer look at their surroundings, especially to detect movement. Then, when the head thrusts forward, the pigeon gains “depth information from motion parallax” (Troje, 2000). Motion parallax is the movement of images as the observer moves; generally, as you move, objects that are closer in distance will seem to move faster than objects at a greater distance. Thus, the thrust phase provides the pigeon the depth information that their vision does not provide (Davie and Green, 1988). Although there are trade-offs for better vision, the pigeons are able to overcome those trade-offs by moving their heads back and forth.
Rock pigeons rely heavily on their vision for survival, which is why it is important for the pigeons to bob their heads in order to enhance their vision. Since the pigeons’ vision lacks depth perception, thrusting their heads compensates for this deficit, giving more visual information of the surroundings to the pigeons in order to see any predators and prey. Additionally, by holding their heads stationary, pigeons can detect movement better than when their heads are moving. They will be able to detect the movement of insects and other small prey so that they can capture the prey and eat. Pigeons will also be able to see the movement of predators, like hawks, so that they may avoid or escape the predator. Indeed, the head bobbing of pigeons is an important behavior for their survival through predator-prey interactions.
Although the head-bobbing seems strange and slightly amusing as if the pigeons were moving their heads in rhythm to a private concert, this behavior is very important for their survival. The head-bobbing strengthens the pigeons’ vision by allowing the pigeons to see the world more clearly in order to spot prey and predators. Next time you see a pigeon, look closely to see its head-bobbing and then, join them by moving your head too.
Catania, A.C. (1964) On the visual acuity of the pigeon. Journal of the Experimental Analysis of Behavior 7: 361-366. doi: 10.1901/jeab.1964.7-361.
Davies, M. N.O. & Green, P.R. (1988) Head bobbing during walking, running, and flying: Relative motion perception in the pigeon. J. Exp. Biol. 138: 71-91.
Frost, B.J. (1978). The optokinetic basis of head-bobbing in the pigeon. J. Exp. Biol. 74: 187-195.
Troje, N. & Frost, B.J. (2000). Head-bobbing in pigeons: How stable is the hold phase? J. Exp. Biol. 203: 935-940.