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Funny Walks: Cranes bob, bob, bob along when hunting

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12:58pm, April 13, 2005
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The jerky neck motions of a whooping crane looking for lunch keep its head still in space about half the time, probably helping the bird to spot food, a video study suggests.

When walking, cranes and many other birds thrust their heads forward, then let their bodies catch up. A Maryland research team now says that it has made the first measurements of a bird's natural head bobbing while hunting. The result fits the idea that head bobbing benefits a hunter, says Thomas Cronin of the University of Maryland, Baltimore County.

Scientists have theorized for decades that head bobbing has something to do with vision, explains Cronin. When a bird's head is still, images form on the retina without the blur of motion. So, the hypothesis goes, maximizing the stillness could help a bird get the best picture of its world, including the telltale wiggle of its food against a complex background. Tests on pigeons, egrets, and terns, among other birds, have supported parts of this idea.

Birds such as gulls don't bob their heads, and their visual systems may be more like those of mammals, says Barrie Frost of Queen's University in Kingston, Ontario. People, for example, use jerky movements of their eyes, rather than their whole heads, to stabilize images.

In work reported in the April 12 Current Biology, Cronin and his colleagues took a look at whooping cranes, the tallest birds in North America. Cronin's research team scattered mealworms and other treats on the ground of enclosures at Maryland's Patuxent Wildlife Research Center in Laurel and videotaped the cranes foraging.

The researchers analyzed, frame by frame, the positions of the head, legs, and body's center of gravity. The cranes advanced in a way that created head stability some 50 percent of the time.

That's about the same proportion that a pigeon walking on a treadmill immobilized its head in experiments by Nikolaus Troje, also of Queen's. The similarity between the birds is "not surprising," he says.

Graham Martin of the University of Birmingham in England welcomes the work on cranes but cautions, "We would need to see this [proportion of steadiness] in quite a few more species before it looked like a pattern." The big, slow-moving cranes can become a model for further work, he adds.

Frost speculates that the head's motion phase may be as important as the still period. He points out that although motion may blur details, it's a great way to learn about the distance of various objects. Bobbing birds, he says, may be alternately detecting prey and determining distance.

Birds' bobbing could provide advantages beyond versatile vision, Frost says. For example, maximizing head stillness might partly camouflage a bird as it stalks food. Frost says that researchers are beginning to use videos having such high speeds that they may reveal whether flying insects also stabilize their heads.

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