Stemming the Tide of Shorebird Losses
With migrations that can span thousands of miles, Pacific shorebirds are among nature's most amazing aerialists. But without crucial stopover habitat along their way, they could be doomed.
Thousands of red knots approach the salty shallows from afar, moving continuously in perfect unison. As the flock drifts upward and downward, probably inspecting the rolling waves, it looks like a gigantic feathered quilt flapping gently in the spring breeze. The birds—a different subspecies than those that migrate along the U.S. East Coast—finally join the others on the mudflat and eagerly commence feeding. Lining up on a dike that flanks the saltpans is a group of equally eager bird researchers from China, Australia, New Zealand, and the United Kingdom.
“Aha, here he is,” cries out David Melville, part of a surveying team from Fudan University led by Ma. He and his colleagues are surveying shorebirds along the coast of the Yellow Sea, from its southern end in Shanghai to its northern tip in the Yalu Jiang estuary. Through his powerful spotting scope, he locates a bar-tailed godwit with a little white flag and colored rings attached to his legs. “I captured and banded him six years ago in Golden Bay [in New Zealand]. It’s so nice to see him again,” Melville says, like reuniting with an old friend. “He has been coming here each spring,” says Matt Slaymaker, who has surveyed banded shorebirds in Bohai Bay every year since 2010 with Global Flyway Network, an organization that aims to monitor and understand changing shorebird populations worldwide. This is common for shorebirds, a phenomenon that scientists call site specificity—that is, they go to the same staging location every year. The trait makes them extremely vulnerable if such places are destroyed.
The importance of these sites came into fuller focus in 2007, says Warnock, when satellite tracking revealed the full migratory cycle and the epic journey of bar-tailed godwits (an Audubon priority species, in significant need of conservation efforts). Most scientists had assumed that these long-distance migrants stopped at a series of islands to rest and eat. But the tracking study, led by Robert Gill, a research wildlife biologist emeritus at the U.S. Geological Survey, stunned the world. All 17 godwits tagged in New Zealand with satellite transmitters followed a three-part route: First they flew 6,300 miles nonstop in a week from New Zealand to the Yellow Sea; after packing on weight for six weeks, the birds departed for Alaska, traveling an additional 4,200 miles in one go. Then, at the end of the breeding season, they flew directly back to their wintering ground, nonstop for 7,300 miles.
Subsequent research using either satellite transmitters or a different kind of tracking device called a geolocator shows that the bar-tailed godwit has company; other flyway shorebirds, such as whimbrels, ruddy turnstones, and bristle-thighed curlews, also fly several thousand miles nonstop on their migrations. Such studies “have not only revealed the impressive odysseys of these avian athletes but also crystalized the importance of the Yellow Sea,” says Yang, who is part of Piersma’s team. “This is where they recover from one super-marathon and prepare for the next. If the sites are gone, then they are doomed.”
At first glance, mudflats are hardly inspiring. They’re uniformly gray and dull and, at low tide, can stretch for miles to the horizon without any landmarks other than puddles. But “their bleak appearance is deceptive,” says Yang, pushing a white cylinder into the mud. When she lifts it, a core of mud pops out. As the sediment goes through a sieve, a plethora of creatures emerge—worms, clams, shrimp, snails, slugs, and crabs. A square meter of mud in Bohai Bay typically contains 50,000 tiny gem clams, the red knot’s favorite food, in spring. “It’s an extremely productive system,” she says.
But “not all mudflats are the same,” says Piersma. A number of conditions, such as currents, topography, and sediment size, must coincide to make a productive tidal area. In addition, shorebirds prefer beaches with an extremely gradual slope, so that the slightest shift in wind or tide exposes vast areas of mudflat. There are only 16 such regions in the world that serve as critical shorebird habitats. “The Yellow Sea is one of the largest and the most productive,” he says.