Birds of a Feather
Scientists are gleaning surprising insights into the feeding, flight and other behaviors of shorebirds that come together in large flocks
VISIT FLORIDA’S MERRITT ISLAND National Wildlife Refuge in winter, and—with a little luck—you can gaze across its shallows and mudflats and see dunlins, western and least sandpipers, sanderlings, short-billed dowitchers, greater and lesser yellowlegs, semipalmated and black-bellied plovers, American avocets, marbled godwits, whimbrels, willets, red knots and the ubiquitous killdeer. All at the same time. And all in an area no bigger than a football field.
Though they breed on tundra or in boreal forests of the Far North, where they are as territorial as any bird, during the winter months and spring and fall migration, shorebirds congregate in great numbers—from thousands to hundreds of thousands, even millions. Wetlands with abundant prey and a water level that is just right are the ecological magnet that draws them together: Seventy percent of the world’s 217 species of shorebird—49 of which occur regularly in North America—feed in water of 4 inches or less.
How do shorebirds of so many species gather together without competing for the same resources? “You can think of this as you going to the grocery store,” says University of Connecticut biologist Margaret Rubega, who has spent more than 20 years studying how birds feed. “When you are in the produce aisle there are a lot of other people there, but they are not exactly like you. So even though you are all in the same place, you might leave with brussels sprouts and spinach and somebody else is going to go out the door with avocados and limes.”
In much the same way, different shorebird species make use of different “shelves” of food within the same area and use diverse techniques to capture prey. The least sandpiper, for instance, has a short, pointed bill and most often remains on the mudflats pecking at small invertebrates it locates by sight. Plovers, with their large eyes and short, stubby bills, also feed by sight. Some have special approaches: The semipalmated plover runs short distances across the mudflats and stops suddenly, freezing with one foot ahead of the other and raised at an angle. The toes, trembling slightly, just touch the surface—a motion that encourages tiny invertebrates to scatter, revealing themselves in the process.
A few feet away, in shallow water, dunlins probe the muddy substrate and capture small clams, worms, insect larvae and amphipods by detecting their movement with special receptors in their bills. Farther out, short-billed dowitchers, whose bill length is more than half again as long as the dunlin’s, wade in deeper water and probe more deeply where they can get at prey beyond the reach of dunlins. Black-necked stilts, the slender, long-legged ballerinas of the shorebird clan, have bills roughly the same length as short-billed dowitchers, but rather than probe the mud at the bottom they walk slowly and pick at prey on the surface of the water. The American avocet, roughly similar in size and shape to the stilt, sweeps its upturned bill side to side just below the water’s surface, filtering out small invertebrates.
In the mid-1990s, Rubega and her colleagues discovered a remarkable feeding technique in phalaropes that has since been found in many sandpipers. These birds use the surface tension that holds a drop of water together to retrieve drops that contain tiny food items, trapped like figurines in a snow globe. The bird works the droplets up into its bill and squeezes them, releasing the liquid and leaving only the food.
Equally amazing, some shorebirds can detect “pressure gradients” caused by prey buried in the mud, an ability first noticed by Dutch biologist Theunis Piersma. “When you watch a red knot,” Rubega explains, “you notice that it puts its beak in the mud, takes it out, then shoves it back in the same place.” In this way the bird builds up pressure on the water between the sand grains much as we do when stepping on damp sand and creating a wet spot around our feet. The red knot has pressure sensors on its bill that detect how much effort it requires to displace the interstitial water. “If there is something in the substrate near the place the red knot probes,” Rubega notes, “especially something hard like a clam, which is very often what knots are looking for, the water between the sand grains builds up, and the knot is able to measure and interpret the changing pressure landscape.”
Another, more recent, discovery comes from researchers working in the extensive mudflats of Roberts Banks near Vancouver, British Columbia, a vital gathering place for western sandpipers. Using high-speed video recording, Tomohiro Kuwae of Japan and an international scientific team found that sandpipers are doing much of their feeding in the biofilm—a dense but paper-thin layer that lies on the surface of the mud. Microorganisms are bound up in this mucouslike biofilm, which holds them in place so they are not washed away by the tides.
As they feed, shorebirds seem completely focused on their foraging, but they are simultaneously on the lookout for approaching raptors. Here, too, the birds’ propensity to congregate in flocks benefits them. There is safety in numbers as many birds watch the sky from countless vantage points. Even at rest, with their beaks tucked under their scapular feathers, their heads half-buried, sandpipers often keep open the exposed eye that is pointed skyward. Many hawks and falcons, after all, have also migrated south for the winter.
Feeding shorebirds chatter and peep companionably, but if a raptor approaches there is a sudden and complete silence followed by a more high-pitched sound as the birds race upward, the loosely knit feeding flock quickly assembling into the unified flight flocks that we find breathtaking—and attacking raptors find confounding. A flock twists and turns, swells and contracts. It may ripple like a flag or undulate like ocean swells. Often the birds bank one way seemingly all at once, exposing their pale undersides, then turn back, showing their darker backs, and sometimes they flip several times so quickly they produce a stroboscopic effect.
It is easier to explain why shorebirds fly in tight formations than how they accomplish it. “People have been wondering about this for millennia,” says biologist Frank Heppner of the University of Rhode Island. One explanation in the 1930s involved “thought transference” and “collective thinking.” And in the early 1970s Heppner himself, who has been studying the phenomenon for 35 years, wrote a paper postulating that the birds were communicating by some kind of neurological radio waves even though there was no clear evidence for it. “We were just looking for something,” he says.
In a 1983 laboratory experiment, biologist Wayne Potts, now at the University of Utah, determined the average reaction time of a sandpiper to be 38 milliseconds. Yet in a frame-by-frame analysis of film footage of sandpiper flocks taken in the field, he discovered that waves of movement traveled from bird to bird in just 15 milliseconds. But that still was not fast enough to account for the synchronized flight maneuvers of flocks. In what has become known as “the chorus-line theory,” Potts proposed that the birds, much like Radio City Music Hall Rockettes performing a high-leg kick, observe “the approaching maneuvre wave and time their own execution to coincide with its arrival.”
A couple years later, computer scientist Craig Reynolds, then of Symbolics Graphics, approached the problem from a different angle, proposing that complex behavior could result from birds instinctively following a few simple rules. He developed a computer model called “boids” that simulated coordinated flock flying. In it, Reynolds devised three rules for each bird to follow: steer to avoid collisions, try to match the speed and direction of nearby individuals, and aim for the average position of others in the flock. He caught the attention of Hollywood before that of biologists: In 1992, Batman Returns used Reynolds’ program to coordinate digital flocks of bats and penguins.
Further progress stalled, Heppner says, because biologists and computer scientists were reading journals only in their own fields. Eventually he and Reynolds crossed paths, as did others who were studying the phenomenon. Most recently, a team of scientists from several disciplines studied flocking starlings in Rome using high-speed stereoscopic photography. “One of the things that makes this whole field of study so interesting,” Heppner says, “is that it’s so difficult—just from a technical standpoint. If you’re going to take three-dimensional pictures of birds and you’re 200 meters away ... well, that’s hard to do.” Expensive cameras, a technician to calibrate the cameras and a powerful computer, among other challenges, make the Rome project something that won’t likely be duplicated with shorebirds any time soon. Results from the study, however, have advanced the understanding of coordinated flying. One interesting observation was that individual birds seemed to respond to no more than six or seven of their immediate neighbors. The scientists wondered if this stems from the cognitive capacities of birds, pointing out that studies of pigeons indicate they only distinguish up to six different objects.
“We still don’t know how birds fly in these formations,” says Heppner, “but we’re getting a lot closer. Whatever the answer, it is a beautiful thing to see. When we know the answer, I’ll still go out and just watch them. Someone once said that understanding that a rainbow is caused by the refraction of light through raindrops does not make you appreciate the rainbow any less.”
A professor of English at the University of Central Florida, Don Stap is the author of A Parrot Without a Name and Birdsong: A Natural History.
By flying in flocks, red knots can travel an average of 3 mph faster than can individual birds.