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.
The abundant food in the mudflat, however, is available only to those with the tools to access them. Shorebirds have developed highly specialized systems—bills, sensory organs, and digestive tracts—to feed on tidal flats. The length of their bills determines how deep they forage in the mud. Some find their prey by touch, others by detecting the vibration of moving animals. Then there are the red knots, which locate their food remotely by sensing changes in the pressure waves they send out when they poke their bills into wet sand. Since most shorebirds swallow their catch whole and crush it with their muscular stomachs, they’re particularly fussy about the size and type of prey they eat. “Many shorebirds simply cannot feed efficiently in habitats other than mudflats,” says Ma.
Driving around Nanpu and other parts of Luannan County is like navigating a gigantic maze. The coastal waters are crisscrossed with seawalls that divide numerous saltpans and shrimp ponds as well as bridges that connect the coast to an increasing number of manmade islands. Between 1994 and 2010 development gobbled up 34 percent of the intertidal mudflats in western and northern Bohai Bay—about 168 square miles. “This has squeezed migratory birds into an ever-smaller remaining area,” says Yang. Her studies—partly supported by the International Crane Foundation—show, for instance, that the number of red knots in Luannan increased from 13 percent in 2007 to 62 percent in 2010 of the total flyway population, which at the same time has been declining by 5 percent to 9 percent a year. Other spring staging species, such as curlew sandpipers, broad-billed sandpipers, and sanderlings, and wintering populations, like Eurasian curlews, are also cramming into the region.
Luannan has therefore become the last resort for both migratory and wintering bird populations. The red knot will be hit worst: 75 percent of the flyway population depends on the coastline along Bohai Bay. Since the 1990s the total number of red knots along the flyway has been declining. This is puzzling because their breeding success in the Arctic and survival rate in wintering grounds in Australia and New Zealand “seem adequate,” says Piersma, whose studies on the flyway have been made possible by BirdLife Netherlands and WWF Netherlands. “There are a lot of concerns about what’s going on.”
The distribution and survival of wintering red knot populations in Australia and New Zealand have been studied for decades and are relatively easy to pinpoint. But estimating their survival rate during migration is tricky. The birds, which weigh about 3.5 ounces—one-third the weight of a godwit—are too small to carry satellite transmitters. So the researchers have turned to old-fashioned re-sighting studies. As a result of work supported by the Australasian Wader Studies Group and Miranda Naturalists’ Trust, a New Zealand-based charitable organization, more than 1,000 red knots have been banded. Since 2009 the Global Flyway Network, headed by Piersma, has been systematically counting them at Bohai Bay during the entire spring migration season, noting how many arrive and when they leave. Combined with results from other sites, the researchers have found signs of a decreasing survival rate during the birds’ time away from Australia. They’re still analyzing the data, “but preliminary findings do point to a strong ‘Bohai effect,’ ” says Piersma.
Several common greenshanks circle over the shallow waters, chirping their characteristic three-note call: teu-teu-teu. A flock of bar-tailed godwits suddenly takes to the air, while the rest are unperturbed, moving away from the dike as they forage in the receding tide. Despite the flurry of adjacent industrial development, the scene is surprisingly peaceful. “How long will it last?” asks Ying Chen, a Ph.D. student of Ma’s at Fudan University. Her surveys are providing a new benchmark to compare with those from a decade ago, and to assess the impact of ongoing land reclamation.
The worst fear for researchers and conservationists is that South Korea’s Saemangeum disaster is duplicated in China. Saemangeum’s once vast, fertile tidal mudflats were one of the most important staging sites for shorebirds along the flyway. When South Korea undertook this massive land reclamation project there was intense domestic and international criticism from scientists and environmental groups. But that didn’t stop it from damming the entire estuary with a 21-mile-long seawall, converting 180 square miles of mudflats and 68 square miles of sea shallows into a desert.
“The impact on the shorebirds has been devastating,” says Nial Moores, director of the Busan-based conservation group Birds Korea. There were large declines in shorebird populations when the seawall was under construction. Then, within weeks of its completion in 2006, millions of mollusks perished and populations of the birds that rely on the mudflats as a staging site decreased sharply. A study co-led by Moores shows a decline of 137,000 water birds, some of which are shorebirds—including 90,000 great knots, nearly a quarter of the global population—from Saemangeum between 2006 and 2008 on their northward migrations. The other 18 most numerous species were also badly hit, including a 91 percent drop for spoon-billed sandpipers.
Surveys of South Korea’s remaining tidal flats showed no evidence that water birds lost to Saemangeum had relocated elsewhere in the country. The closure of the seawall was soon followed by “a sudden drop in wintering populations of almost every species in Australia, especially great knots,” says Danny Rogers, chair of the scientific committee of the Australasian Wader Studies Group.