Shorebirds "Defy Gravity" to Eat, Study Says

May 15, 2008

Certain types of shorebirds defy gravity to get morsels of food up their long, slender beaks into their mouths, new research shows.

The birds, called phalaropes, exploit the same principle that allows water droplets to stick on windows.

Phalaropes take advantage of this effect, called surface tension, by using a tweezering motion with their beaks to "ratchet up" droplets embedded with tiny bits of food.

"In the absence of the ratcheting, the drop would just be stuck there," said John Bush, a mathematician at the Massachusetts Institute of Technology in Cambridge.

"But by continually opening and closing the beak, they can actually propel it upward."

Bush is the lead author of a paper describing the mechanism in tomorrow's issue of the journal Science.

Feeding Frenzy

Phalaropes start their meals by spinning in circles on the surface of the water. The motion creates an upwelling that draws suspended crustaceans and other tiny invertebrates within reach of the birds' beaks.

The birds then peck at the surface of the water, capturing their prey embedded in small droplets. But since their beaks are pointed down, the birds must overcome gravity to get the food into their mouths.

Bush and colleagues found the birds accomplish this by opening and closing their beaks rapidly, each step propelling the drop closer to their mouths.

When the beak closes, the edge of the drop closest to the mouth shoots even closer while the other edge stays put. When the beak opens, the trailing edge catches up while the leading edge stays put.

"This works in the presence of gravity to overcome gravity," Bush said.

In the Details

Margaret Rubega is a biologist at the University of Connecticut in Storrs who first discovered that phalaropes exploit surface tension in their feeding. But she only had a vague notion of how the birds' jaw-spreading affected the mechanism.

"This is very exciting for me to have these detailed physics worked out at a smaller level, because it explains some of the actual biology of the animal," she said.

Bush noted the mechanism hinges on how easily water beads up on a given surface—in this case, the surface of a phalarope's beak.

Oily water doesn't bead well, making animals that use this feeding mechanism vulnerable to oil spills, he noted.

"You think in terms of an oil spill mucking up their feathers and so preventing them from flying," he said. "But our study also makes clear that they won't be able to eat."

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