The Dunlin is among the most cosmopolitan and well studied of all the small sandpipers. It is a familiar species throughout the year, either in its striking breeding plumage of black belly and rufous back (hence its previous name, Red-backed Sandpiper) or during winter when it is gray and nondescript but occurs in flocks of thousands or tens of thousands.
As many as nine races of Dunlin breed in the Holarctic, three of them in North America, where this monogamous, territorial species breeds on subarctic and arctic coastal tundra from southwestern Alaska north and east to James Bay, Canada. During winter it occurs mostly on large estuaries along the Pacific and Atlantic coasts of the United States and northern Mexico; some Alaska birds spend the winter in coastal East Asia.
Clams, worms, insect larvae, and amphipods figure prominently in the diet of this species, reflecting its tie to coastal and intertidal areas throughout most of its annual cycle. In some areas such as the Central Valley of California and states bordering the Gulf of Mexico, substantial numbers of Dunlin move inland from the coast in midwinter.
The Pacific Coast population (C. a. pacifica), numbering about half a million birds, is substantially larger than the other two North American populations. Mortality in all populations appears to be greatest during winter, particularly from avian predators such as falcons. Despite the Dunlin's broad geographic range, populations of several races appear to have declined in recent decades, perhaps because of continued loss and degradation of wetland habitats.