PRBO Conservation Science
Quarterly Journal of PRBO Conservation Science, Number 162, Fall 2010: The Water Issue


Yellow, Blue, and Green -- Warbler, Water, Willows

At Risk: Migratory Songbirds' Nesting Habitat

Mark Dettling

California's Water Legacy
CEO's Column
A Shorebird Saga
Willows and Warblers
Profile of Dave Shuford
PRBO Highlights
TomKat Ranch
Len and Patti Blumn
Staff Migrations

A Yellow Warbler in its typical habitat. Photo by Tom Grey.
Each spring, millions of migratory songbirds that winter in Central and South America travel thousands of miles to their North American breeding grounds. Yellow Warblers are among these neotropical migrants, which have just a few short months to raise the next generation before making the return trip. In this annual ritual that perpetuates their species, the quantity and quality of breeding habitat available to the birds is of critical importance. That's where water comes in--especially in California, where Yellow Warblers are even more closely tied to watercourses than in other parts of their range.

Any description of the Yellow Warbler's habitat preference invariably will first mention water and willows. In 1923, esteemed California naturalist William Dawson observed that "...the Summer [Yellow] Warbler breeds wherever willows are found, and this means that his summer home in California is nearly confined to... the vicinity of water, whether running or stagnant.' Willows--water-dependent plants--provide great places for the warblers' nests and harbor the insect prey needed for rearing young.
The Yellow-billed Cuckoo is a secretive bird inhabiting streamside habitats. It is listed as endangered in California. PRBO began a major effort in 2010 to survey for this species along the Sacramento River, one of the cuckoo's last strongholds in the state. The data we collected will help guide conservation planning efforts. Photo: Jerry Oldenettel / Flickr Creative Commons.

Are California's willow thickets, and thus warbler populations, at risk? Historically, Yellow Warblers bred across the entire state, wherever proper habitat existed. Willow patches were abundant then, due to the scouring action of regular flooding in riparian (streamside) ecosystems. In the past 100 years, widespread loss of riparian vegetation and massive water diversions have led to population declines for Yellow Warblers and other riparian-dependent species such as the Least Bell's Vireo, Willow Flycatcher, and Yellow-billed Cuckoo.

The changing climate means that all such denizens of willow groves in the West face a risky future. Warmer and drier conditions across much of California will make the remaining riparian habitats important refuges for more and more native wildlife.1 Conservation of riparian systems today is a hedge against loss of biodiversity in the West tomorrow. And the Yellow Warbler is a bellwether of effective protection, enhancement, and restoration of riparian habitat.

For several decades, PRBO has been guiding and monitoring restoration projects large and small. When water was returned to the creeks that feed Mono Lake in the 1990s, PRBO researchers found that Yellow Warblers, nearly extirpated in the area, came back in droves. At the San Joaquin River National Wildlife Refuge, a growing population of warblers has spread into recently restored areas. Documenting nesting songbirds enables us to provide even more effective recommendations for our partners' on-the-ground actions.

A key to successful restoration is encompassing natural river processes, which are compromised by massive water diversions from our rivers. For example, the natural cycle of flood events would normally contribute to the diversity of habitat along a river, benefiting the Yellow Warbler as well as a myriad of other plants and animals. A hallmark of river systems is their dynamic, changeable condition.

The warming climate poses significant additional challenges to conserving rivers and streams, not only for wildlife but also for the benefits they provide to society, such as replenishing aquifers and nurturing fisheries. We can expect to see the amount and location of rainfall and snowpack altered. Snow-fed rivers will have less water, and seasonal streamflows will occur earlier. In the Central Valley, climate change will impact water availability directly by changes in rainfall and indirectly by management decisions designed to capture and store water for human consumption.

Today's conservation efforts must take into account future climatic conditions and the uncertainty inherent in those conditions. One way to address this uncertainty is to prioritize and restore significant reaches of riparian, to reduce the environmental stress in an already compromised system--and give ecosystems and Yellow Warblers a fighting chance.

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