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Each spring and fall, an estimated 1 billion birds migrate through the Pacific Flyway, which snakes down from Alaska, along the West Coast of the United States and Mexico, and into South America. Species such as the Tree Swallow, Marsh Wren, and Black-throated Gray Warbler all use this migratory corridor as they make their way to their southern overwintering grounds in fall and their northern breeding grounds in spring. Where many of these birds stop along the way and in what numbers, however, has not been thoroughly studied.  

Now new research reveals what has been long-suspected but never confirmed: California’s Central Valley and the Colorado River Delta are hotspots for North America’s migratory landbirds. The study, published today in , shows that tens of millions of birds depend on the regions’ river and wetland habitat on their journey through a largely dry, arid landscape. 

While the research team had a hunch that the Central Valley and the Colorado River Delta were important migratory stopover sites, “there really wasn't any kind of science or data to back up those claims,” says William DeLuca, lead author of the study and a migration ecologist for ���Ϲ���’s Migratory Bird Initiative. “I don't think we realized just how important they were to landbirds during migration.”

Although the importance of these areas to various waterbirds and other resident species has long been known, the new findings show that these regions are also bottlenecks for dozens of migratory species. In California, the Central Valley and its diverse array of habitats slices down the middle of the state, flanked by the Pacific Coast Range to the west and Sierra Nevada Mountains to the east. The Colorado River Delta in Northwestern Mexico is a smaller region—about a sixth of the size of the Central Valley—with a braided network of estuaries and wetlands that empty into the Gulf of California.

To discover which species visit which regions during their migrations, the research team used data from eBird, a community science app that allows birders to note when and where they spot a species. This data, in combination with a mathematical model developed by , also allowed the team to estimate the total of birds visiting each region for the first time. “It’s allowed people like us to start to ask these really cool questions in these novel ways that we never would have been able to ask—or we could ask, but we can never answer,” DeLuca says.

Before the scientists could determine how important these landscapes are to migrating landbirds, they first narrowed their search down to 112 species that they knew were likely to use the regions for refueling, overwintering, or breeding. They divided the two landscapes into five study territories: the Colorado River Delta, and the four in California’s Central Valley—Sacramento, Yolo, San Joaquin, and Tulare. They then compared eBird data from the study areas to data from nearby regions of the same latitude, stretching from the west coast to the Continental Divide. 

The data revealed that a vast number of migratory species funnel through California's Central Valley and the Colorado River Delta on their journeys. In total, around 17 million landbirds used the Colorado River Delta in the fall, and 14 million used the Delta in the spring. The four Central Valley regions hosted approximately 65 million migratory landbirds in the spring and 48 million in the fall. 

For certain birds, the regions proved particularly significant. More than 30 species had greater than one percent of their continental population migrate through the Central Valley in the spring and fall season. “If more than one percent of the species population uses that site, then we know that that site is really important to that species at the population level,” DeLuca says.

Another surprising finding was that more than a quarter of the continent’s Tree Swallow population—the most abundant migrant in the San Joaquin and Tulare study regions—used the Colorado River Delta, an area the size of the island of Hawai’i. “It kind of knocked our socks off,” DeLuca says. “We were like, ‘Whoa! is this is this right?’”

The Tulare region, the southernmost section of the Valley, saw the largest total number of visiting species. There, roughly 80 percent of the Lawrence’s Goldfinch population passes through during spring migration—a finding that "shocked" Andrea Jones, co-author of the study and director of bird conservation for  

“These migratory pathways are ingrained in birds, and they are sort of still following them even though there's a fraction of the landscape available that used to be there,” Jones says. She points out that before water was diverted for agriculture and human use, the region was still arid, but it was also home to many more rivers and wetlands than exist today. Loss of these habitats packs an extra punch for Western migratory landbirds, which Jones says face a different set of challenges than their Eastern counterparts. “Unlike in the East Coast, these birds are crossing vast stretches of very arid areas and they're looking for green patches.” 

Western birds are spared the overwater flights that many Eastern birds are tasked with, but their challenge is navigating dry landscapes with few resources. And with climate change, water use, and habitat loss reshaping these landscapes, Jones says these birds are under extra strain. California’s Central Valley has lost much of its grasslands, rivers, and wetlands as human development and agriculture have expanded. 

Mexico’s Colorado River Delta has faced similar challenges, says Osvel Hinojosa-Huerta, director of the Coastal Solutions Fellows Program at the Cornell Lab of Ornithology, who was not involved in the study. Upstream dams have diverted the river’s flow, leaving parts of the Delta dry for much of the past two decades, says Hinojosa-Huerta. Though the Colorado River water levels are predicted to , restoration efforts have proved successful in some areas.

“In the last 20 years, there has been that shift from a dying ecosystem, or a delta was considered dead, to an area that provides hope,” Hinojosa-Huerta says. He notes that allocating water resources for conservation can be challenging when people and the agriculture industry also depend on that water, but in some cases, land can be irrigated with more water-efficient approaches, and water can be reused or returned to the environment when possible. 

For DeLuca, the value of the team's findings isn't just knowing where the birds go, but also showing the importance of the continued conservation of these vital habitats. Despite how dramatically the landscapes have changed, the study shows that birds still rely on what resources remain along their journeys. He hopes this work can point to the areas most in need of protection before more species are pushed to their limits.

“These are really altered landscapes, but they're still super, super important,” Deluca says. “Any information that we can use to attach a higher importance to those locations and to get more people interested only increases the probability that we can have more effective conservation.”