With a population estimate of approximately 3.5 million individuals (Bishop et al. 2000), the Western Sandpiper is one of the most abundant shorebirds in the Western Hemisphere, despite its restricted breeding range in coastal tundra regions of western Alaska and far-eastern Siberia.
The majority of the global population spends the boreal winter in coastal bays and estuaries along the Pacific coast of the Americas between British Columbia and Peru, although at least 10% winters along the Atlantic coast and in the Caribbean between New Jersey and Venezuela. During northward migration in April and May, huge, spectacular flocks numbering hundreds of thousands of individuals are common at major stopover sites from San Francisco Bay to the Copper River Delta in Alaska. Most Western Sandpipers migrate along the Pacific coast, although significant numbers move through the continental interior, particularly on the southward migration.
During winter, Western Sandpipers are latitudinally segregated by sex, body size, and age at first breeding. A higher proportion of females are found in the southern part of the wintering range, while males predominate in the north. Southern birds are larger overall and have disproportionately longer wings and bills. Northern-wintering juveniles are likely to attempt breeding in their first summer, while southern-wintering juveniles are likely to delay northward migration and breeding until their second summer.
The breeding biology of Western Sandpipers has been extensively studied in the Yukon-Kuskokwim Delta (Holmes 1971, 1972, 1973, Ruthrauff and McCaffery 2005, Johnson and Walters 2008, Johnson et al. 2008, 2010, Ruthrauff et al. 2009) and Seward Peninsula in Alaska (Erckmann 1981, Sandercock 1998a, b, Sandercock et al. 1999, 2000), providing a wealth of information on the species' biparental breeding biology and reproductive success. Others have investigated Alaskan breeding populations on the North Slope (Gates et al. 2012) and around Kotzebue Sound (Schamel and Tracy, unpubl. data), and a Siberian population in Chukotka, Russia (Tomkovich and Morozov 1983, Lappo et al. 2012).
On the breeding grounds, Western Sandpipers eat insects and their larvae as well as aquatic and marine invertebrates present along the edges of tundra ponds and coastal lagoons. In coastal areas during migration and the non-breeding season, they forage on marine invertebrates such as crustaceans, bivalves, and polychaete worms. A recent discovery is that Western Sandpipers and other small shorebirds also graze on biofilm, a surface matrix of diatoms, microbes, organic detritus, and sediment present in intertidal habitats.