As many as 1.1 million shorebirds have been observed at one time using the Copper River Delta during the peak migration (April 25 - May 15). Western Sandpipers and Dunlin, the two most abundant species of shorebirds on the Pacific Coast, stop here each year before flying further north and west to their breeding grounds. Their presence on estuarine mudflats behind the barrier sandbar islands is, by far, the most impressive use of Copper River habitats by shorebirds. Obtaining the necessary energy reserves on the Delta greatly influences their reproductive success elsewhere in Alaska and/or Siberia. Important too are the upland marsh wetlands for other shorebird species breeding here.