Tens of millions of birds in the western U.S. migrate along the same two corridors, according to a new study from the Audubon Society.

California‘s Central Valley and the Colorado River Delta along the US-Mexico border are major thoroughfares for dozens of species, including hummingbirds, swallows and goldfinches.

The research indicates they host ‘major proportions of many species’ global populations,’ including 80 percent of Lawrence’s goldfinches.

The birds rely on wetland habitats to see them through a long journey over a dry and harsh landscape.

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Some 48 million landbirds, including nearly 40 percent of all Anna's Hummingbirds, use California's Central Valley as a stopover when migrating south in the fall, according to a new report from the Audubon Society

Some 48 million landbirds, including nearly 40 percent of all Anna’s Hummingbirds, use California’s Central Valley as a stopover when migrating south in the fall, according to a new report from the Audubon Society

‘The Valley and Delta act as migratory bottlenecks,’ the Society said in a statement. ‘As birds migrate through these latitudes, they funnel into these study regions.’

Those dense concentrations make the birds vulnerable, it added, and both regions should be the focus of conservation efforts.

Scientists had long suspected the two regions were important stopovers, but ‘there really wasn’t any kind of science or data to back up those claims,’ Bill DeLuca, a migration ecologist with National Audubon Society’s Migratory Bird Initiative, told Audubon magazine.

‘I don’t think we realized just how important they were to landbirds during migration.’

To get an answer, DeLuca’s team analyzed data from eBird, a citizen-science app that allows birdwatchers to record when and where they’ve spotted a new feathered friend.

They combined that data with a mathematical model developed by the landbird conservation group Partners in Flight to get an estimate of all the birds touching down in both areas.

‘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,’ said DeLuca, lead author of a new study in Ornithological Applications.

‘Or we could ask, but we can never answer.’

More than a quarter of North American tree swallows migrate through the Colorado River Delta in northwest Mexico, a habitat threatened by upriver dams and climate change

More than a quarter of North American tree swallows migrate through the Colorado River Delta in northwest Mexico, a habitat threatened by upriver dams and climate change

Unlike migrating birds in the eastern US, which fly over water, western birds spend much of their journey crossing arid landscapes and honing in on lush spots. 

Situated between the Sierra Nevada mountains and the Pacific Coast Range, California’s Central Valley is a 450-mile expanse that includes grasslands, wetlands, rivers, and other diverse habitats.

Northwestern Mexico’s Colorado River Delta is a patchwork of estuaries and wetlands that feed into the Gulf of California.

DeLuca’s team carved out five sectors— the Delta and regions of the Central Valley in Sacramento, Yolo, San Joaquin and Tulare — then focused on 112 species known to frequent the two corridors.

They determined some 48 million landbirds used the Central Valley as a stopover when heading south in the fall, including nearly 40 percent of Anna’s Hummingbirds.

In the spring, 65 million migrating birds stopped in Tulare alone, including 80 percent of Lawrence’s Goldfinches who frequented Tulare alone.  

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According to DeLuca, if even more than one percent of a species uses a region ‘then we know that that site is really important to that species at the population level.’

The Colorado River Delta, which is about a sixth of the size of the Central Valley, hosted about 17 million migratory birds in the fall and 14 million in the spring, including more than a quarter of North American tree swallows.

‘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,’ said co-author Andrea Jones, director of bird conservation for Audubon California.

The Delta and Central Valley are part of the Pacific Flyway, a bird ‘superhighway’ that’s one of four major North American migration routes.

It’s long been known both were vital to local species and waterbirds, ‘but we were astounded at the sheer number of land-based birds who also depend on the region,’ said Meghan Hertel, director of land and water conservation for Audubon California.

Both corridors have been vastly transformed by humans: The Central Valley’s grasslands and wetlands have largely been gobbled up by commercial and agricultural development.

And while the United Nations Educational, Scientific and Cultural Organization (UNESCO) designated much of the Colorado River Delta a nature reserve in 1993, upstream dams and climate change have left parts of it dry for decades.

‘These are really altered landscapes, but they’re still super, super important,’ Deluca said.

‘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.’

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A 2019 study indicated the number of birds in North America has plummeted by nearly three billion in the past 50 years, the equivalent of losing one in four birds.



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