The millions of waterbirds that migrate each spring from South America to as far as the Arctic can't do it in one trip. They stop to rest and refuel several times along the way to survive the grueling journey.
But widespread land-use change has shrunk the area of stopover habitat available to ducks, geese, shorebirds, and other migratory species. In central California, concerned citizens, scientists, and conservation groups have joined forces to protect what remains.
Scientists from Point Blue Conservation Science and the Nature Conservancy (TNC) combined satellite imagery and statistical models with farmer incentive programs and the efforts of hundreds of volunteers contributing data through a citizen science app to pinpoint the areas of central California with the greatest potential for providing migratory bird habitat. They recently published their analysis of the success of these incentive programs in maintaining bird habitat during an extreme drought sustained between 2013 and 2015 across the western United States.
"Before this research was completed, we had a sense that these programs were succeeding in offsetting the impacts of the drought on wildlife, but now we know exactly how critical they are in providing bird habitat in the Central Valley," lead author Matt Reiter, principal scientist and quantitative ecologist at Point Blue, said in a statement.
Retaining Habitat in a Transformed Landscape
Shorebirds, including sandpipers and stilts, dunlins, and dowitchers, feed on aquatic invertebrates that live in mud or wet sand, so they seek wetlands during their stopovers.
California's Central Valley, once home to a vast system of about 6,250 square miles of wetlands, is one such key stopover region for migratory shorebirds and waterfowl along the Pacific migratory flyway.
The valley extends more than 250 miles north to south and up to 62 miles east to west. Massive agricultural development has eliminated more than 90 percent of the naturally occurring wetlands, leaving the birds dependent on flooded agricultural fields for food during their stopovers.
California's water is highly managed, so anthropogenic factors play a large role in determining when and where the impacts of drought appear on the landscape. A pair of financial assistance programs provided farmers in the birds' flight paths with incentive payments to flood their fields at key times during the 2013–15 drought to create habitat for migrating waterbirds.
The area's rice growers flood their fields each fall with two to four inches of water in preparation for the next year's harvest. The flooding converts the paddies into ideal migratory shorebird habitat, so TNC's BirdReturns and the Natural Resources Conservation Service's Waterbird Habitat Enhancement Program offer farmers financial incentives to flood their fields for one to two additional months, to coincide with the bird migration.
Avid birdwatchers across the Central Valley helped the partners to identify the areas to target for habitat management and confirm these areas' use by target species. Cornell Lab of Ornithology's eBird online platform encouraged birdwatchers in the region to submit their bird observations to its database, before and after the incentive programs began.
eBird statisticians compiled the observations from hundreds of birders to build models that predicted where 25 species of shorebirds would likely be present across the Central Valley during their spring and fall migrations. The models generated maps that showed when and where the target species were expected to gather. Overlaying maps of bird concentrations with the distribution of surface water indicated mismatches—areas where management action, in the form of flooding, was most needed.
Seeing Water From Space
To assess the success of these programs, the researchers used satellite imagery to examine the impact of the drought on the timing and extent of surface water in the Central Valley.
"By using satellites to track habitats regularly," Reiter told Mongabay, "we can look for hotspots of change and use that information to help prioritize conservation actions."
They analyzed Landsat images from 2013 to 2015 to identify areas of open water (more than 30 percent vegetated) across the Central Valley and to measure the distribution of open water habitat in managed wetlands and fields of rice, corn, and other crops between July and May.
Using data from ground and aerial surveys, they developed predictive models to identify open water, separate from saturated soil underneath thick vegetation. "We will probably not get moist soil without some ponded water with our model," Reiter said. "That said, because we track water year round, we can identify those places that maintain some open water across months."
The models quantified the influence of drought, precipitation, season, region, and protected status on the proportion of open water in each land cover type between July and May of the following year.
The scientists then calculated the relative contribution to available habitat during that period of the two farmer incentive programs. They used the image data to estimate the daily proportion of flooded habitat in each type of field (e.g. rice, corn, etc.) that was provided by these programs. They multiplied that proportion by the total amount of the crop planted in each year (834 square miles in 2013 and 655 square miles in 2014) to get the area of open water habitat made available by the programs each day.
"We [then] combined predicted shorebird abundance values with predicted wetland extent to identify times and locations where temporary wetlands could deliver potentially high-value shorebird habitat," said co-author Mark Reynolds, lead scientist for the Nature Conservancy's California Migratory Bird Program.
The analysis showed that the drought substantially reduced the availability of open water habitats across the Central Valley's fields and wetlands, both spatially and temporally. During the drought, the amount of open water habitat decreased by 40 to 80 percent, compared to non-drought years, and the decline varied by land cover type, time of year, and region. For example, corn and wetlands in the San Joaquin Basin dried out more than rice and wetlands in the Sacramento Valley. Protected wetlands retained more water than unprotected, privately owned wetlands.
It also revealed that the incentive programs provided a large portion of the open water in rice fields during the fall and spring waterbird migrations in the drought years.
"BirdReturns provided 39 percent of the post-harvest flooded rice during the fall, when flooded habitat is at its lowest and waterbirds are in high abundance," Reiter said in a publication summary. "And the WHEP created 64 percent of the habitat during the winter. Overall, incentive programs provided 35 percent of the habitat on the landscape October through March."
Possibly more important for hungry migratory birds during a drought, the BirdReturns program provided up to 61 percent of all available flooded rice habitat on certain fall days and the WHEP created up to 100 percent of available habitat on some days during the winter.
Do Birds Use the Managed Wetlands?
The crowd-sourced eBird observations collected after the incentive programs began helped to verify the use by the birds of different types of managed areas. The data showed that crop fields participating in the BirdReturns pilot program, for example, hosted far more target migratory waterbirds than control fields (with no additional flooding).
"This new approach to rent habitat on demand promises to engage more farmers to provide habitat in a flexible manner that can be tailored to ever-changing weather patterns and farming practices," Reynolds said.
The birdwatchers recorded more than 220,000 birds representing 57 species in the BirdReturns fields, with February through March shorebird densities 20 times higher than on non-participating fields. These totals included more than 20,000 dunlins, representing roughly 20 percent of the entire overwintering dunlin population in the Central Valley.
"The study highlights the role incentive programs can play in species conservation," Reiter said. "Program managers should place a high priority on maintaining incentive programs in the face of more frequent severe droughts in order to sustain waterbirds in the Central Valley and the Pacific Flyway."
The survival of millions of migratory birds in increasingly modified landscapes now depends on human intervention, so assessing the success of specific actions can help managers apply them elsewhere.
"Open water and wetlands are critical habitat resources across the world and have seen some of the greatest losses to human development," Reiter said. "Our models could be used in other landscapes where water and wetlands play a key role in supporting wildlife habitat to prioritize those places and times when we need to make sure to sustain those water dependent ecosystems and habitats on the landscape."
This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.