Scientists uncover migration bottlenecks that are used by tens of millions of birds

Scientists from the National Audubon Society have published a study highlighting the high conservation value of California’s Central Valley and Colorado River Delta for tens of millions of migratory land birds each year.

The study, led by Audubon’s new Migratory Bird Initiative and published in Ornithological Applications, estimates that more than 65 million birds use large parts of the Central Valley during fall migration and approximately 17 million birds use the Colorado River Delta during spring migration. These regions are important migration routes at the population level for dozens of species. Audubon scientists also found that the Central Valley and the Colorado River Delta are migratory “bottlenecks” because these regions had highly concentrated numbers of birds compared to similarly sized regions at the same latitude. Protecting the Central Valley and the Colorado River Delta will be critical to the future conservation of North American migratory bird populations.

“The migration seasons in spring and autumn can be the most dangerous times of the year for migratory birds. We now know more about bird migration than ever and can take important steps to protect it, ”said Dr. Bill DeLuca, migration ecologist at Audubon’s Migratory Bird Initiative and lead author of the study. “It is hard to overstate the importance of protecting birds all year round, and not just in breeding and wintering areas, if we are to secure their future.”

Anna’s Hummingbird, Copyright Mark S Szantyr, from the Surfbirds Galleries

Using data from the Cornell Lab of Ornithology’s eBird Status and Trends team and population estimates from Partners in Flight, Audubon scientists summarized the frequency and spatial concentration of 112 species of migratory land birds weekly in the study areas.

Audubon researchers highlighted three key findings:

1) California’s Central Valley and the Colorado River Delta along the border between the United States and Mexico are migration foci for tens of millions of migratory birds that fly up and down in western North America.

2) The tens of millions of birds that fly through these regions make up a large part of the global populations of many species. For example, in the spring, more than 27 percent of tree swallows in North America migrate through the Colorado River Delta and 80 percent of Lawrence’s goldfinches migrate through the Tulare region of the Central Valley. In the fall, more than five percent of Black-throated Warblers migrate through the Colorado River Delta, and nearly 40 percent of Anna’s hummingbirds migrate through the Central Valley. Full list of types and proportions on request.

3) The valley and the delta act as migration bottlenecks. When birds migrate through these latitudes, they end up in these study regions. These high bird concentrations make them particularly vulnerable to threats. Efforts and research to protect migratory birds should therefore focus on these key regions.

Audubon California, the state office of the National Audubon Society, has worked for decades on efforts to protect land and water in the Central Valley.

“We have long known how important the valley is to waterfowl migration, but we were amazed at the sheer number of land birds that are also dependent on the region. For example, almost 40 percent of all Anna’s hummingbirds migrate through the valley in the valley fall. This study puts an exclamation mark on what we have already seen as the hemispherical importance of the valley, ”said Meghan Hertel, director of land and water conservation at Audubon California.

The Central Valley and Colorado River Delta are important not only to locally breeding birds, but also to common migratory birds that people enjoy in their backyards and parks up and down the Pacific Flyway.

“This study further demonstrates the overwhelming importance of the Colorado River Delta, particularly for the many birds that migrate, including 27 percent of the world’s tree swallows in the spring and more than five percent of the black-throated-gray warbler brood in the fall, as do many other land birds, ”said Jennifer Pitt, Audubon’s program director for the Colorado River. “And while the delta is just a remnant of what it used to be – due to upstream water diversions, historical drought and climate change – we know that targeted and strategic restoration projects with our partners in Mexico will produce positive results for humans and birds . “

Many North American land birds are declining at alarming rates, and we need to know the places they depend on all year round. Audubon’s Migratory Bird Initiative works with migration researchers and conservation professionals to harness the best available migration science to steer conservation efforts towards these critical landscapes.

To learn more about Audubon’s migratory bird initiative, please visit


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