The Value of One Valley

On January 26, 2021, a team of scientists from the National Audubon Society published a new study in the journal Ornithological Applications proving what many birders, naturalists and ornithologists have long suspected: California's Central Valley provides critical habitat for many North American migratory landbirds.

The value of Central Valley habitat for waterfowl and shorebirds has long been realized.  The Valley's formerly vast expanses of wetland still provide critical overwintering habitat for millions of migratory waterfowl, including ducks, geese and swans, along with Sandhill Cranes and a number of wading birds and shorebirds.  Other studies have focused on the value of breeding habitat in the Valley for riparian nesting birds.  But until this study, it had not been proven, conclusively, that the Valley also provides critical stopover habitat for a wide array of migratory landbirds as well, a broad group of birds that includes swallows, flycatchers, vireos, warblers and other songbirds.

Tuolumne River, Stanislaus County

Previous studies have already shown that migratory bird populations are declining in North America (numbers of spring migrants have decreased by 14% since 1970, according to a recent study), and research such as this helps to identify specific regions and habitats that those birds use during migration.  Once these areas have been identified, conservation efforts can be concentrated on the most valuable regions, allowing migratory birds to continue to move successfully between breeding and overwintering habitats.

In addition to the Central Valley, the study also examined the value of the Colorado River Delta, determining conclusively that it too provides critical habitat for migratory landbirds.  And of course, both of these regions have been highly altered and degraded over the last century, largely through the draining of wetlands and the conversion of riparian floodplains to agriculture.  These alterations have caused huge losses to both the extent and function of habitat.

Riparian habitat along the Tuolumne River, Stanislaus County

The study focused specifically on 112 species of landbirds, and found that greater than one percent of the entire North American population of over 30 species of landbirds move through the Central Valley during spring and fall migration.  One percent might seem low, but it actually represents quite a significant proportion of a species' population.  For some species, the percentage of the population using the Valley is much higher: 80% of all Lawrence's Goldfinches, for example, use the Valley during their spring migration, and nearly 40% of North America's population of Anna's Hummingbirds migrate through the Valley during the fall.  (Admittedly, these are both western and largely Californian species.)  Almost unbelievably, a quarter of North America's Tree Swallow population migrates through the relatively small Colorado River Delta.

According to the study, approximately 65 million landbirds migrate through the Central Valley during the spring, and around 48 million move through during the fall.  

Additionally, an estimated 14 million landbirds migrate through the Colorado River Delta during the spring, and 17 million move through during the fall.

Wetlands at Merced National Wildlife Refuge


The annual migration of birds is a fascinating topic of study.  Here in California, birds move along the Pacific Flyway, which stretches thousands of miles along the Pacific coast from the Arctic to South America.  Along this flyway, birds avoid the long overwater crossings embarked upon by birds that use the central and eastern routes.  

But here in the arid West, birds face the challenge of navigating through a highly fragmented mosaic of largely arid habitat.  In the West, wetlands and riparian corridors become essential lifelines for migratory landbirds.  And those found in the Central Valley are of utmost importance to birds due to the nature of California's topography.  Bounded by the long, high ridge of the Sierra Nevada and the arid Great Basin desert to the east, and the vast Pacific Ocean to the west, the Central Valley acts as a bottleneck, concentrating migrating birds and funneling them through its 400-mile length.

Perhaps the most urgent message to emerge from this study is the great need to continue to protect and restore habitat in the Central Valley and Colorado River Delta.  

These regions may look, to the untrained eye, like they have been altered beyond hope.  But this study proves that despite their altered and diminished state, habitat in the Valley is still widely used and heavily relied upon by a wide range of migratory birds.  

For conservation efforts to be effective, they must focus not only on breeding habitat, but habitat used during migration and winter as well.  Studies like this one give weight to the arguments and causes of environmental organizations working to save and restore wetlands, grasslands and riparian areas up and down the Central Valley.  

Anyone can look at a habitat and say, hey, I think this riparian area should be protected because it looks like good habitat for migratory birds, but if a study like this one can be attached to a specific region, scientifically proving its great value, there is a much greater chance of being able to save that area in the future.

We can now say with confidence that habitat in California's Central Valley is, conclusively, valuable at the population level for migratory landbirds.  

This means that the health of the North American population of these species depends, at least in part, on the health of habitat in the Central Valley, and the continued loss of Valley habitat poses potential threats to the entire populations of these species.

Wetlands at San Joaquin River National Wildlife Refuge, Stanislaus County


I ought to mention that the researchers who worked on this study used data from the citizen science database, eBird, to reach their conclusions.  This is just one example of why eBirding matters!  When you see birds, no matter how ordinary they seem to you, make the effort to accurately report the sightings to eBird.org and contribute to valuable scientific studies like this one.  

Your data can help save habitat and the birds that depend on it!


Read the full research article, The Colorado River Delta and California's Central Valley are critical regions for many migrating North American landbirds.

Or, read an article about the study from Audubon.org.

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