MIT solves gravity-defying bird beak mystery

Shorebirds take advantage of surface tension to capture prey

CAMBRIDGE, Mass.--As Charles Darwin showed nearly 150 years ago, bird beaks are exquisitely adapted to the birds' feeding strategy. A team of MIT mathematicians and engineers has now explained exactly how some shorebirds use their long, thin beaks to defy gravity and transport food into their mouths.

The phalarope, commonly found in western North America, takes advantage of surface interactions between its beak and water droplets to propel bits of food from the tip of its long beak to its mouth, the research team reports in the May 16 issue of Science.

These surface interactions depend on the chemical properties of the liquid involved, so phalaropes and about 20 other birds species that use this mechanism are extremely sensitive to anything that contaminates the water surface, especially detergents or oil.

“Some species rely exclusively on this feeding mechanism, and so are extremely vulnerable to oil spills,” said John Bush, MIT associate professor of applied mathematics and senior author of the paper.

Wildlife biologists have long noted the unusual feeding behavior of phalaropes, which spin in circles on the water, creating a vortex that sweeps small crustaceans up to the surface, just like tea leaves in a swirling tea cup.

The birds peck at the surface, picking up millimetric droplets of water with their prey trapped inside. Since the birds point their beaks downward during the feeding process, gravity must be overcome to get those droplets from the tip of the bird's long beak to its mouth. Until now, scientists have been puzzled as to how that happens.

Scientists speculated that the feeding strategy depended on the drop's surface tension. Surface tension normally dominates fluid systems that are small relative to raindrops (for example, the world of insects), but it was not clear how it could benefit shorebirds. A key observation was that in order to propel the drop, the birds open and close their beaks in a tweezering motion.

To unravel the mystery, Bush teamed up with Manu Prakash, a graduate student in MIT's Center for Bits and Atoms, and David Quere, of the Ecole Polytechnique in Paris, a visiting professor in MIT's math department at the time of the study. They built a mechanical model of the phalarope beak that allowed them to study the process in slow motion.