To travel the many thousands of miles between summer Arctic breeding grounds and winter sites in the Southern Hemisphere, millions of shorebirds make pit stops on the mudflats of the Yellow Sea between China and South Korea. But as the mudflats are filled in and developed—because of dams, rising sea levels, and the construction of sea walls—the birds most dependent on this stopover are declining, in some cases by as much as 8% per year, researchers report today in Nature Communications. Biologists analyzed data collected by citizen scientists between 1993 and 2012 about the numbers of 10 bird species that reach New Zealand and Australia. The numbers in species such as the gray-tailed tattler that don’t rely heavily on the mudflats remained about stable over that time, they discovered. But for the seven species of which at least 60% need to make this pit stop, numbers declined. Those species in trouble include the eastern curlew (see photo), the curlew sandpiper, and the great knot. In contrast, the analysis found that total migration distance, body size, nonbreeding grounds location, and other factors have not contributed to this decline. Given the critical importance of these stopovers, the researchers call for greater international efforts to slow the mudflat loss. Already, 36 governments and other organizations have formed the East Asian-Australasian Flyway Partnership, but they may not be able to act in time to save these birds, the researchers note.