California’s Central Valley and the Colorado River Delta Are Epicenters for North America’s Migratory Birds

A database called eBird reveals as many as 65 million birds fly through these Western migration zones

A pink and green hummingbird is drinking nectar from a red flower
During fall migration, nearly 40 percent of Anna’s Hummingbirds (Calypte anna) migrate through California’s Central Valley Becky Matsubara via Flickr under CC BY 2.0

Migratory birds have followed the same flight patterns for millennia, searching for abundant food resources. The journey is often risky, and birds undergo harsh weather patterns—from storms that can throw them off course to dry arid landscapes that provide little to no food resources.

A new study published this week in Ornithological Applications found tens of millions of birds depend on the river and wetland habitats weaved within the Colorado River Delta and California’s Central Valley while they make their journey across the dry western landscapes, reports Corryn Wetzel for Audubon.

These two regions were suspected as essential stopover sites by ornithologists in the past. But not enough evidence supported this idea until data collected from the community science app eBird showed otherwise. eBird is an app where anyone from seasoned birders to casual novices can note when and where they saw a bird species. Scientists use the data collected from eBird to track species populations, spot trends, and trace habitat use.

Using eBird along with a mathematical model created by Partners in Flight, the researchers determined that more than 65 million birds travel through California’s Central Valley during the spring migration and 48 million during the fall, reports Yale’s Environment 360 Digest. About 17 million birds pass through the Colorado River Delta, an area approximately the size of Hawai’i, during the spring, and 14 million birds pass through during fall migration.

The eBird data was narrowed down to 112 species of birds that researchers knew used the regions knew used the regions during migration. A significant portion of a bird species’ entire population flies through these Western regions. The data found that in the spring, more than 27 percent of North America’s tree swallows migrate through the Colorado River Delta, and an astonishing 80 percent of Lawrence’s goldfinches migrate through the Central Valley, according to a National Audubon Society statement. In fall, nearly 40 percent of Anna’s hummingbirds migrate through the Central Valley.

“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,” says Williams DeLuca, a migration ecologist and lead author of the study, to Audubon.

The Delta and Valley, while crucial to various species of birds, has been altered in the past by human development, agricultural expansion, and diverted river flow. The changes in the ecosystem have birds under extra strain from the already challenging journey, but the data shows that despite the alterations, the birds continue to depend on these landscapes’ reports, Audubon.

“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,” Andrea Jones, director of bird conservation for Audubon California and co-author of the study, tells Audubon.

Deluca and his team hope that the data will inspire conservation efforts for these habitats before the bird species are pushed to their limits.

*Editor’s Note, February 3, 2021: A previous version of this article incorrectly stated that 82 million birds use the regions for migrations, a combination of how many birds use both locations. However, because millions of birds use both locations, adding the figures together is inaccurate and 65 million birds is a more accurate count. Similarily, due to a copy error, a previous version of this article stated 12 species of birds use the area when in fact 112 species use thee area. The story has been edited to correct that fact.

Get the latest stories in your inbox every weekday.