The Nature Conservancy is testing a new program, called BirdReturns, that rents habitat rather than buying it. The program pays rice farmers to leave water in their fields for a few extra weeks at critical times during the birds’ winter and spring stopovers in the valley—effectively creating “pop-up,” on-demand wetlands. The Sacramento Valley is the northern half of California’s vast Central Valley, which once included some 4 million acres of wetland habitat. But over the past century, the landscape has been retooled for farming and other uses. Today, just 250,000 acres of wetlands remain, largely protected in federal and state wildlife refuges. These remnants still function as vital winter resting grounds and migratory pit stops. The Central Valley provides habitat for near two-thirds of the Pacific Flyway’s ducks and geese and one-third of its shorebirds, which pause here to pack on fat before they head north again to breed each spring.