A method to estimate migrant shorebird numbers on the Copper River Delta, Alaska

We estimated the: annual population of Western Sandpipers (Calidras mauri) and Dunlin ( Calidris alpina pacifica) stopping over on the Copper River De lta during peak spring migration 1992-1995. Our calculations required four components: total daily shorebird numbers, the daily proportion of each spe cies, average length of stay, and the detection probability. For the 21-d p eriod 26 April-16 May, annual population estimates for Western Sandpiper ra nged from 1.2-4.1 million birds per year. Dunlin estimates for this same pe riod 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 su rveys are needed to detect a 10% decline in Western Sandpiper numbers. Base d on the proportion of birds in the Pacific Flyway stopping over on the Cop per River Delta, we estimated the Western Sandpiper Pacific Flyway populati on was >2.8 million in 1992 and >4.3 million in 1995. These data indicate t hat the Copper River Delta continues to support the largest spring concentr ation of shorebirds in the Western Hemisphere.