Shorebird Use of the Lower Colorado River Delta During Migration

The natural and anthropogenic wetlands of the Colorado River Delta are recognized as critical stopping and staging areas for migrating shorebirds. Given the importance of these wetlands and the uncertainty around their water sources, we conducted 9 paired, aerial and ground surveys between April 2021 and April 2023 to quantify migration season shorebird use across a study area of 486 km2 that covered the lower Colorado River estuary and the Ciénega de Santa Clara. We estimated that an average of 272,515 individual shorebirds occupy the study area at any given time during the active migration season. Shorebirds were most abundant in mudflats and flooded playas of the Ciénega de Santa Clara, followed by wetlands of the lower estuary. Shorebird abundance was positively related to the proportion of a count site covered by mudflats and flooded with surface water, and was positively related to the proportion of a year that a count site was flooded with surface water. While 21 shorebird species were recorded during ground surveys, 8 species made up more than 90% of the total birds counted. Estimated average abundances for these species were: 103,132 Western Sandpiper; 31,763 Red-necked Phalarope; 30,067 Long-billed Dowitcher; 23,527 American Avocet; 20,196 Least Sandpiper; 19,197 Black-necked Stilt; 10,901 Snowy Plover; and 8,993 Long-billed Curlew. We speculate that the study area hosts at least 1% of the North American population for 11 shorebird species during migration. Results confirm the importance of these wetlands for migrating shorebirds and highlight the need for securing water sources given increasing human water demands and decreased water availability due to climate change.