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The northbound phalaropes passed through a few weeks ago. We saw them at the Hayward Regional Shoreline, a couple hundred at least, spinning around like little feathered tops in one of the fenced ponds. Every few minutes a portion of the flock, seized by some apprehension, would take off, circle, and touch down on the water again.
These were mostly red-necked phalaropes, and at this season, unlike the gray-and-white birds of winter, they lived up to their names. It was easy to distinguish the brighter-colored females from the duller males. After a lot of time on the spotting scope, I was able to pick out a red phalarope among the rednecks, a bit larger and bulkier and vivid brick red on the neck, breast, and belly—probably a female. All I needed for the Phalarope Trifecta was a Wilson’s, but I couldn’t summon one up.
I’ve written here before about the atypical behavior signaled by those colors. Like only a few other birds (jacanas, painted snipe,), all three phalarope species have socially dominant females that take the lead in courtship and territorial defense, and males that incubate the eggs and rear the chicks after the female has moved on to a new mate.
Darwin thought of sexual selection as a process of female choice shaping male ornamentation, like the tail of the peacock. In phalaropes, it must have been male preferences that produced the gaudy female plumage. Maybe a female phalarope’s bright colors are cues to greater reproductive fitness or low parasite loads, as appears to be true of gaudy males in other avian species. Another project for someone in need of a dissertation topic.
This piece isn’t about sexual selection, though. It’s about biomechanics. I have never been at all mechanically inclined, but I’ve always been fascinated about the way things work. Not that every adaptation is perfect; there are a lot of jury-rigged panda’s thumbs in the natural world. But evolution, in most cases, has done a splendid job with the material at hand. I remain impressed by the feet of the gecko, and now I find out that ivy roots also use nano-scale forces to stick to smooth walls.
I mentioned the phalaropes spinning in circles. One bird was clocked at 57 rotations per minute. They do this to concentrate small aquatic organisms, as humpback whales build bubble nets to corral fish. Phalaropes, known to sailors as “bowhead birds,” hang around whales to feed on their leftovers. There were no whales at the Hayward Shoreline, but apparently plenty of small tasty creatures.
But how do the phalaropes handle their prey? The first hint came from ornithologist Margaret Rubega in the 1990s. Watching film footage of a whirling phalarope, she noticed a dark blur moving up the bird’s beak. Although the phalarope appeared to be ingesting seawater, its beak wasn’t built for suction.
The mechanism wasn’t clarified until this year, when John Bush, a mathematician at MIT, MIT graduate student Manu Prakash, and David Quere of the Ecole Polytechnique in Paris published an article in Science. They built a stainless-steel replica of a phalarope beak and put it through a series of lab tests. With the technobeak in a vertical position, water leaked out, as did drops of silicone oil.
Then the scientists tried opening and closing the beak at a range of angles. Open it too wide and a water droplet splits; too narrow and it spills. But there’s one point-the sweet spot of the phalarope beak-at which the water drop bends, and surface forces push it toward the bird’s mouth, in defiance of gravity. Bush, Prakash, and Quere say the key process is contact angle hysteresis, the force that causes raindrops to stick to windowpanes. Normally it creates resistance to droplet motion; in this case it promotes it.
As Rubega and others had noticed, feeding phalaropes constantly tweezer their beaks open and shut. The motion exploits surface tension to ratchet a food-laden droplet up the beak and into the mouth. When the beak closes, the leading edge of the drop moves toward the mouth; when it closes, the trailing edge follows suit. Bush and his co-authors call the process the “capillary ratchet.”
Shape is critical: this only works with a long, narrow beak.
Prakash, whose department is the Center for Bits and Atoms, is looking at practical applications: microfluidic devices that allow controlled stepwise motion of microliter droplets. Not as dramatic as gecko Velcro, but a benefit nonetheless.
The MIT group also sounds a cautionary note. If a phalarope encounters an oil spill and its beak becomes oil-soaked, the capillary ratchet doesn’t work. “Once they feed through that thin slick of oil, they are done,” says Rubega. How that translates into phalarope losses during spills is a huge unknown. These are sparrow-sized birds that spend much of the year on the open ocean.
We may never notice the casualties.