Head-bobbing behavior in foraging whooping cranes favors visual fixation

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 [1–3]. Gaze stabilization could be required for successful visual search, particularly for moving objects [4], 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. The whooping crane is the tallest North American bird, making their head movements easy to observe; their eyes are as much as 1.5 m above the ground. It feeds omnivorously on small animals as well as berries, seeds and small tubers [5]. Our experiments took place in large outdoor pens at the whooping crane facility of the US Geological Survey's Patuxent Wildlife Research Center near Laurel, Maryland. Sufficient data for the desired analyses were collected over several days of work with four adult-sized cranes, of both sexes. A mounted, digital video camera recorded movements of foraging cranes. Head, body and leg positions were retrieved from videotapes using a graphical overlay on each successive frame (Figure 1A). Recordings were analyzed only when cranes travelled perpendicular to the camera's line of sight. To standardize among birds, speeds were recorded as 'Hock lengths per s' (Hl s –1). The hock is the long ankle portion (containing the tarsometatarsus bone) of the leg, about 300 mm long in adult-sized whooping cranes. Figure 1B illustrates a typical data sequence. The body moved forward at a fairly constant speed, rising over each planted foot. The head was stabilized six times during the sequence (arrows), each time as a leg was lifted and moved forward. As each foot was planted the head rapidly projected forward to the next stable position. In the first series of trials, cranes walked or ran towards desirable food (a piece of fish) tossed in front of them, allowing us to record the birds as they approached an object …