A bird is able to make drops of water defy gravity and flow into its mouth.
A team of MIT mathematicians and engineers has shown that some shorebirds use their long, thin beaks in a tweezering motion to make prey-bearing water droplets rise upwards so they can be consumed.
The work is even more remarkable because last year a team at the University of Bristol, led by Prof Jens Eggers, thought that it was the first to make droplets flow up a slope, by vigorously vibrating the droplets, and announced the feat in the prestigious journal Physical Review Letters.
But now it seems that birds beat them to this gravity defying feat, probably by millions of years.
As Charles Darwin showed nearly 150 years ago, bird beaks are exquisitely adapted to the birds’ feeding strategy.
In this case the north American phalarope 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 team reports in Science.
Wildlife biologists have long noted the unusual feeding behaviour 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 tiny droplets of water with their prey trapped inside.
Since the birds point their beaks downward, 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.
To unravel the mystery, Prof John Bush and colleagues built a mechanical model of the phalarope beak that allowed them to study the process in slow motion.
As the beak scissors open and shut, each movement propels the water droplet one step closer to the bird’s mouth.
In this stepwise ratcheting fashion, the drop travels along the beak at a speed of about 1 meter per second.
The mechanism depends 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 Prof Bush.
This gravity-defying action is made possible by the surface tension of water, as well as a physical effect known as “contact angle hysteresis,” which normally causes drops to stick to solids.
When combined with the tweezering motion of the beak, however, this effect enables the water droplets to rise mouthward, explained Prof Bush.
