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On an average day during migration, some 250,000 shorebirds can be found in the wetlands of the Colorado River Delta—twice previous estimates. This is according to new science from the National Audubon Society and Pronatura Noroeste, published today in Waterbirds: The International Journal of Waterbird Biology.
The study found that, for at least 10 shorebird species, the number of birds in the Delta on a typical day during migration was at least 1 percent of their entire hemispheric population, a key benchmark for designating a site as a Landscape of Regional Importance by the Western Hemisphere Shorebird Reserve Network. For some shorebird species, the total percentages across an entire migration season are staggering. Factoring in the occasional turnover of migrating individuals, it is possible that the Delta supports 12 percent of the global Western Sandpiper population, 20 percent of the world’s American Avocets, and an astonishing 24 percent of Long-billed Dowitchers and Long-billed Curlews.
Once a vast floodplain sustained by the mighty Colorado River, the Colorado River Delta in the middle of the Sonoran Desert in Mexico has long been a vital lifeline for millions of birds—breeding, wintering, and stopping over on their epic journeys across the Western Hemisphere. While its significance to landbirds has only recently been revealed, the Delta has long been recognized as one of the hemisphere’s most critical stopovers for migratory shorebirds.
This study—with support from the LightHawk Conservation Flying and Jonathan Vargas Vega (aerial observer)— also suggested that, after accounting for turnover, more than a million shorebirds may pass through the Delta each season. This solidifies the Delta’s place among North America’s most critical migratory stopovers for this imperiled group—a group that, according to the 2025 State of the Birds Report, has more Tipping Point species than any other in North America.
The findings are both a triumph and a warning. They affirm the Delta’s enormous ecological value while highlighting its fragility. Once nourished by a river that flowed freely to the sea, more than 90 percent of the Colorado River’s delta habitats have been lost to upstream dams and diversions, long-term drought, and an increasingly arid climate. Today, shorebirds rely heavily on wetlands sustained by agricultural runoff and a very small volume of environmental water releases supplied by the United States, Mexico, and a binational coalition of non-governmental organizations. As global change tightens its grip, the future of these habitats—and the birds that depend on them—hangs in the balance.
Counting Shorebirds by Sky and Ground
To collect data for the study, shorebird counters conducted nine airplane surveys between 2021 and 2023 to reveal the full scope of the Delta’s importance. They flew over key wetlands and estimated shorebird abundance while ground teams recorded and verified species composition and habitat conditions. Aerial surveys offered a sweeping view, while ground observations at select sites provided a critical correction factor for species composition across the broader survey area.
The results confirmed what conservationists had long suspected: shorebirds concentrate in areas rich in mudflats and shallow water, particularly in the southern portion of the Ciénega de Santa Clara where brackish water mixes with the tides before draining into the Upper Gulf of California. These findings reinforce the urgent need to protect the inflows that supply these wetlands. Without a reliable source of water, these critical habitats—and the countless birds that rely on them—could vanish.
A Fragile Stronghold
Historically, the Colorado River’s delta was a vast wetland system of nearly two million hectares that ebbed and flowed with seasonal snowmelt from the Rocky Mountains to the north. Today, more than 90 percent has been lost. The Ciénega de Santa Clara—at 40,000 acres the Delta’s largest remaining wetland—survives thanks to an unlikely source: agricultural wastewater from the United States, which provides 90 percent of its water.
“For all its ecological value, the Ciénega exists in constant uncertainty,” said lead researcher Tim Meehan. “It’s a thriving wetland, but its water source is not protected. That makes it highly vulnerable to changes in water management policies.”
One threat that has loomed for years is the possible reactivation of the Yuma Desalting Plant, a U.S. facility designed to treat the agricultural water runoff that supplies the Ciénega and deliver it to irrigate farms. The Yuma Desalting Plant, built in the late 1980’s, was operated for less than two years and today is a long-abandoned facility with old technology that would take millions of taxpayer dollars to repair and more to operate. If operations restarted, the water that feeds the Cienega would be diverted to this plant with important implications for shorebirds and their habitat.
Securing a Future for Shorebirds
Shorebirds don’t recognize borders. Many species that depend on the Delta breed in the Arctic migrating through North America and wintering as far south as Central and South America. Protecting them requires collaboration across international boundaries, from breeding grounds to stopover sites to wintering habitats.
One model for such cooperation is Minute 323, an agreement under the 1944 Colorado River Treaty between the United States and Mexico that has helped restore modest environmental flows to the Delta but does not address flows for the Ciénega. While a critical step, experts warn that much more must be done to ensure the Delta’s wetlands endure.
“This research makes it clear that water for wildlife needs to be a priority in future management decisions,” Meehan said. “A quarter-million shorebirds on any given day during migration—and potentially a million over a season—depend on it.”
According to Stefanny Villagomez, wildlife biologist at Pronatura Noroeste, the study’s findings highlight a critical opportunity.
“The results of this study provide key data to help promote regional water management policies focused on conservation, which would secure the future of this vital ecosystem for the survival of migratory shorebirds, especially given the current human water demands and climate change,” Villagomez said. As human demands and climate pressures intensify, the Colorado River’s fate grows ever more uncertain. Without sustained water, the Delta’s wetlands will continue to shrink—with them, one of the Western Hemisphere’s most critical waypoints for migratory shorebirds.
For these remarkable travelers, the Colorado River Delta is far more than a rest stop—it is a keystone in their migratory journey. Safeguarding its future means preserving the interconnected web of life that depends on this vanishing river.
For more information on shorebird migration, please visit the Bird Migration Explorer.
