Ground-based censuses of shorebirds were conducted during the winters of 2000–2008 (even years only) and the springs of 2002–2004 and 2008 at Estero Santa Cruz, Sonora, México, a 3,622 ha coastal wetland at the Gulf of California between the Colorado River Delta and Bahía Santa Maria and Ensenada Pabellones, Sinaloa. We documented 21 shorebird species using Estero Santa Cruz. Winter species richness averaged 13 (SD = 2, range 11–16 species, N = 8) while the spring species richness averaged eleven (SD = 2, range 8–16 species, N = 15). In all censuses, Calidris spp.—primarily Western Sandpiper (C. mauri)—was the most abundant species group, accounting for up to 92% of daily abundance—varying between 60–91% in winter and 34–92% in spring. The highest winter daily count of calidrids was 3,485 while the spring high count was 7,095. The two highest single day counts were the latest dates censused (10 April 2004, 9 April 2008) suggesting that the peak of spring migration comes later, and thus that peak shorebird abundance in the estero is higher than our censuses have recorded. Estero Santa Cruz and other estuaries geographically intermediate between larger shorebird staging areas and migratory bottlenecks represent important migratory stopover sites. Such sites allow for flexibility in migratory pathways, and thus deserve conservation attention and management.