The Western Sandpiper is one of a group of very similar small shorebirds called "peeps". The Sanderling is obviously larger with a bolder wing stripe. White-rumped and Baird's Sandpiper are larger and appear especially longer-winged. Least Sandpiper is browner, has yellow legs (unless stained by mud), and a slightly decurved bill. Western Sandiper is told from the similar Semipalmated Sandpiper in alternate plumage by its rufous upperparts and in juvenile plumage by its rufous scapulars. In basic plumage and in-between plumages note that the Western often has an obviously longer, thinner-tipped bill with a droop at the tip, a more square-shaped head, and sometimes retains a few dark chevrons on the flanks. The juvenile Semipalmated Sandpiper has a darker crown than the basic plumage Western Sandpiper. The very rare Little and Rufous-necked Stints have rufous in the throat in alternate plumage, and lack the rufous scapulars in juvenile plumage. In basic plumage, they are very similar (see Jonsson & Grant, 1984).
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.