Doing the Bob

A crane's jerky neck movements may allow the bird to see its food more clearly.

You’ve probably seen birds that bob their heads while they walk. These jerky motions may look awkward, but there’s a reason for this jumpy strut, scientists say. It may help the birds see their food.

 

The head jerks of a foraging whooping crane may allow it to see its target more clearly.

 

B. Mackay

Scientists have long supposed that head bobbing in cranes, pigeons, egrets, and other birds has something to do with vision. The idea is that a bird sees details best when its head is steady and makes distance estimates best when its head is moving. Mammals—including people—and probably some types of birds jerk their eyes instead of their heads to achieve the same effect.

To test this idea, researchers from the University of Maryland, Baltimore County studied whooping cranes, the tallest birds in North America. When walking, this long-necked bird thrusts its head forward, then lets its body catch up.

To begin with, the scientists scattered mealworms and other treats on the ground of enclosures at Maryland’s Patuxent Wildlife Research Center in Laurel. Then, they used video cameras to record the behavior of the birds.

For the analysis, the researchers looked at each video frame and recorded the positions of a bird’s head, legs, and center of gravity. The results showed that a crane moved its neck so that its head kept still about 50 percent of the time.

The still period would allow a crane to focus on pinpointing its food. And, although motion would blur details, this period would be a great way to judge distances to objects. Bobbing birds may be alternately detecting prey and determining distance.

Further research might expose other advantages of head bobbing. It’s possible, for example, that keeping its head still for as long as possible between bobs helps a bird sneak up on prey.

With new, high-speed cameras, researchers are now starting to look for head-bobbing behavior in insects, too. As funny as the herky-jerky looks on the dance floor, “doing the bob” may be more common in nature than you might think.—E. Sohn

Going Deeper:

Milius, Susan. 2005. Funny walks: Cranes bob, bob, bob along when hunting. Science News 167(April 16):245-246. Available at http://www.sciencenews.org/articles/20050416/fob6.asp .

For a QuickTime movie showing a crane’s bobbing motion, click here. Courtesy of Thomas Cronin.

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