Bobble-Head Birds See Straight
Anyone who has fed pigeons in the park has probably wondered why they bob their heads forward and back when they walk. It not only looks comical to us, it seems like it would give them a very confused sense of sight. Leave it to scientists to go find out why birds bob their heads. Strangely, nobody seems to have studied this before.
A team of Maryland biologists decided to investigate, and just published their results in Current Biology.1 They found that the head bobbing actually maximizes the amount of time the eyes see a stationary picture. “The stable phase of bird head-bobbing movements is particularly interesting,” they say, “because the behavior, unique to birds, clearly contributes to visual gaze stabilization.” (To make this all sound scientific, they had to give it the highfalutin name “gaze stabilization.”)
Many species of birds move their heads forward through a series of successive, fixed positions when walking. This unique ‘head-bobbing” behavior stabilizes visual fields during body movement, preventing motion blur of the retinal image. Gaze stabilization could be required for successful visual search, particularly for moving objects, but the time available for stabilization varies with walking speed. No direct evidence has been published showing that birds favor the stabilization phase while foraging either for moving or immobile food. We examined head-bobbing behavior in foraging whooping cranes, Grus americana, as they searched the ground for food, and found that they walk at speeds that allow the head to be immobilized at least 50% of the time. This result strongly suggests that, in cranes and probably many other birds, visual fixation via head stabilization is necessary for object detection and identification during visual search. (Emphasis added in all quotes.)
They chose Big Bird the whooping crane because its head, with eyes nearly five feet off the ground, makes the bobbing easy to observe. Watching male and female cranes feed in a pen, they recorded their movements with a video camera, and analyzed the motion mathematically. Think of the two extremes: “At a slow pace, fixation is possible almost all the time, but the area that can be searched is limited,” they write, but “At near-running speeds, much more ground can be covered, but head thrusts dominate the thrust-fixation cycle.” (For the rest of us, that means that while the bird is running, its head would bob so often the vision would be blurry.)
Birds forage at a walking pace that gives them the best of both: a large search area, and stable eyes to detect prey. Their preferred walking pace allows them to see with tripod-steady intervals about 50% of the time, giving them occasions to change their head angle and “fixate new objects of visual interest,” they say. Pigeons bob their heads when landing, they noted, and herons assume a rigid gaze, even if the perch moves under their feet. These observations suggest that a steady gaze is important to a bird. In conclusion, they say, “The observation that foraging whooping cranes favor visual fixation strongly implies that for them, and probably many other species of birds, stabilization of vision is necessary for object detection, recognition, and localization.”
1Kronin, Kinlock and Olsen, “Head-bobbing behavior in foraging whooping cranes favors visual fixation,” Current Biology, Volume 15, Issue 7, 12 April 2005, Pages R243-R244, doi:10.1016/j.cub.2005.03.036.
For this to evolve, the neck muscles had to adapt to fast controlled motion while the brain simultaneously found a way to interpret the motion. Additional structural support might be needed to keep the eyes from popping out. Does the brain tune out the jerky part of the motion and stitch together a seamless set of still images? Perhaps so. It would be interesting to project on a screen how the smart bird brain (see 02/01/2005) interprets its visual input. This story illustrates how even the simplest everyday phenomena in the living world around us have a purpose. Investigating such things with careful observation and analysis often reveals an amazing set of parts working together harmoniously for the big picture.