The Sanderling is a small, plump sandpiper, most commonly associated with sandy coastal beaches, which it occupies in winter and while on migration. When foraging on sandy beaches, Sanderlings move quickly, running ahead of incoming waves and chasing after receding ones, probing the sand for food. To a lesser extent, the species occupies sand- and mudflats, lagoons, and intertidal rocky shores.
Although its numbers are generally small locally, it is probably the most widespread maritime shorebird wintering in North America, found from 50°N (southern British Columbia) and 42°N (Massachusetts) on the Pacific and Atlantic coasts, respectively, to 50°S (southern Chile, southern Argentina). Its wintering range thus spans some 100° latitude, encompassing most temperate and tropical beaches in the Americas.
During the nesting season, this bird occupies very different habitat: high-arctic tundra. Its breeding range is circumpolar, with highest numbers in the Canadian Arctic archipelago, Greenland, and high-arctic Siberia. In North America, it winters in relatively small numbers along the Atlantic, Pacific, and Gulf coasts; most winter in Central and South America. During migration, this species is locally common on all 3 coasts of North America and on the Great Plains, with peak numbers in spring on the Atlantic Coast of Florida and Delaware Bay and in fall on the New England and Delaware Bay coasts.
Sanderlings nest on the ground, typically in exposed, well-drained sites, either alone or in loose colonies. Mating systems appear to vary among populations, ranging from monogamy to serial polyandry. The extent and frequency of double-clutching and polyandry in this species have not been determined. The nest, a shallow scrape sparsely lined with leaves, typically contains 4 eggs. Both sexes incubate, brood, and attend to fledged offspring, although sexual roles depend on the mating system.
During winter, to varying degrees among populations, some individuals of either sex are strongly territorial in the intertidal zone and above the high tide of sandy beaches; otherwise individuals roam in conspecific, non-territorial flocks.
Research on the Sanderling has been focused largely on the nonbreeding season, and the most extensive work has been on foraging behavior, spacing, population structure (Myers et al. Myers et al. 1979b , Myers et al. 1979a , Myers et al. 1981 ; Pitelka et al. 1980 ; Myers Myers 1981a , Myers 1983b , Myers 1984 ), and physiology on migration and the wintering grounds (Castro et al. Castro et al. 1989 , Castro et al. 1992 ; Castro and Myers Castro and Myers 1990 , Castro and Myers 1993 ).
Aspects of social behavior in winter have been well described. But, the Sanderling's mating system, population dynamics, and other aspects of the reproductive season are significantly understudied. Regional populations are in rapid decline, with the apparent cause being habitat degradation and increasing recreational use of sandy beaches.