Bobbing birds do it for a reason

Pronounced head-bobbing behavior during walking is a characteristic of diverse species of birds, but how this behavior benefits the birds and under what circumstances it proves useful have remained uncertain. Researchers this week report findings that strongly suggest that for some birds head bobbing is critical for the stabilization of their visual world, despite the motion of their bodies, and thereby enables the accurate detection of objects such as food items.

The motion associated with the head-bobbing behavior consists of alternate phases of holding the head still and rapidly thrusting it forward during each step. This behavior, found in birds but in no other vertebrates, gives them a vaguely comic appearance but is known to be critical for visual stabilization during body movement. What was not known was whether or not this means of avoiding "motion blur" was essential for high-quality vision in birds; many animals, including humans, stabilize visual fields only transiently, through the use of eye movements alone, and accept quite a lot of so-called "visual flow" when in motion.

In new work addressing this question, Thomas Cronin and Matthew Kinloch at the University of Maryland, Baltimore County, with the collaboration of Glenn Olsen of the Patuxent Wildlife Research Center, observed how foraging whooping cranes control head-bobbing as they search for food in the grass near their feet. The tallest birds in North America--their eyes can be more than 5 feet above the ground--whooping cranes exhibit high-amplitude head movements during locomotion, making the measurement of the speed of the head relatively easy via non-invasive computer-video techniques. The cranes search both for mobile prey (frogs and insects) and for items such as acorns, seeds, and tubers. The research team found that the time devoted to holding the head still decreased with walking speed; at a very slow pace, the head was still most of the time, but when the birds began to run, it never went through a stable phase. The critical observation, however, was that when the birds are searching for food, they walk at a moderate pace that allows the head to be held still more than 50% of the time. In other words, when foraging, whooping cranes favor visual fixation.

This result strongly suggests that visual fixation not only removes motion blur but also plays a critical role in permitting the detection, localization, and recognition of objects. Head-bobbing behavior, so characteristic of birds, permits the close and stable examination of objects in view during the immobile phase and allows the birds to change their point of gaze and probably to gather a sense of space and of the relative positions of objects when the head is thrust forward.

Source : Cell Press