Copyright © 2005 Elsevier Ltd All rights reserved.
Current Biology, Volume 15, Issue 7, R243-R244, 12 April 2005
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Head-bobbing behavior in foraging whooping cranes favors visual fixationCorresponding author
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 [1,2,3]. 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.
The whooping crane is the tallest North American bird, making their head movements easy to observe; their eyes are as much as 1.5m above the ground. It feeds omnivorously on small animals as well as berries, seeds and small tubers . 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’ (Hls–1). The hock is the long ankle portion (containing the tarsometatarsus bone) of the leg, about 300mm 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 of known location. Figure 2displays results for 11 sequences (solid circles). The proportion of time that the head was fixed decreased almost linearly with walking speed (Figure 2A), and the head ceased to be fixed at all when the cranes ran, moving at speeds faster than ~5Hls–1 (~1.5ms–1).
As fixation times decrease with increasing travel speed, the relative time of the thrust phase lengthens. The regression line fitted to data points plotting maximum head speed has a slope near 1.0 (Figure 2B), indicating that the head thrusts forward at a constant speed of about 2Hls–1 relative to the body. Evidently, the thrust mechanism is constrained in its operating range, forcing fixation to become ever briefer as speeds increase — the bird simply cannot turn up the cycle rate proportionately with walking speed.
How do whooping cranes behave during natural foraging? At a slow pace, fixation is possible almost all the time, but the area that can be searched is limited. At near-running speeds, much more ground can be covered, but head thrusts dominate the thrust–fixation cycle. To record the behavior of naturally foraging birds, we simply allowed cranes to search in familiar territory, sometimes after food had been scattered on the ground. Cranes walked at speeds of 1.3 to 2.6Hls–1 (~0.4 to 0.8 m s-1), where fixation can be maintained at least half the time (Figure 2A, open squares). The relationships between fixation time, head thrust speed, and walking speed (Figure 2, open squares) were similar to those of birds approaching a known visual target.
Thus, during visual search, whooping cranes keep their heads fixed most of the time. The stable phase of bird head-bobbing movements is particularly interesting because the behavior, unique to birds, clearly contributes to visual gaze stabilization. Pigeons head-bob when landing [6,7], and herons stabilize their heads rigidly when walking or when their perch moves, almost certainly for visual function [8,9]. Head movements nevertheless play essential roles in vision, giving visual cues for distances and relative locations of objects, providing an opportunity for changes in head angle (see supplemental data), and permitting birds to fixate new objects of visual interest [3,4,10].
Summarizing, our results suggest that in whooping cranes, visual fixation is required during foraging. Fixation time varies with walking speed, but visually searching cranes walk at speeds that permit them to keep their heads (and visual fields) immobilized most of the time while covering a reasonably large search area. 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.
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We thank the crane staff of the Patuxent Wildlife Research Center for their assistance and support and G. Katzir, N. Hempel de Ibarra, and E. Warrant for comments. This report is based on research supported by the National Science Foundation under Grant Number IBN-0235820, to T.W.C.