We estimated the annual population of Western Sandpipers (Calidris mauri) and Dunlin (Calidris alpina pacifica) stopping over on the Copper River Delta during peak spring migration 1992–1995. Our calculations required four components: total daily shorebird numbers, the daily proportion of each species, average length of stay, and the detection probability. For the 21-d period 26 April–16 May, annual population estimates for Western Sandpiper ranged from 1.2–4.1 million birds per year. Dunlin estimates for this same period ranged from 0.3–0.9 million. For both species, numbers were highest in 1993 and lowest in 1994. Power analysis determined that 15 yr of aerial surveys are needed to detect a 10% decline in Western Sandpiper numbers. Based on the proportion of birds in the Pacific Flyway stopping over on the Copper River Delta, we estimated the Western Sandpiper Pacific Flyway population was >2.8 million in 1992 and >4.3 million in 1995. These data indicate that the Copper River Delta continues to support the largest spring concentration of shorebirds in the Western Hemisphere.