Flyway, the sprawling wetlands of the Sacramento Valley are an important stop for migratory birds; however, only a fraction of the historic wetlands remain. Seasonally flooded rice paddies in the Sacramento Valley comprise a large portion of the habitat currently used by migrating birds. Native bird populations have been declining steadily since the 19th century. Species that were once common but now are endangered or gone include the southwestern willow flycatcher, western yellow-billed cuckoo, least Bell's vireo, and warbling vireo. Another reason for dropping numbers are the introduction of non-native species, such as the parasitic cowbird, which lays its eggs in the nests of other bird species causing its hatchlings to compete with the others for food. There were once 9 species of amphibians that used the Sacramento River, but some have become extinct and most other populations are declining due to habitat loss caused by agriculture and urban development. Amphibians originally thrived in the marshes, sloughs, side-channels and oxbow lakes because of their warmer water, abundance of vegetation and nutrients, lower predator populations and slower current. This population once included several species of frogs and salamanders; the foothill yellow-legged frog and western spadefoot are listed as endangered species. Riparian and wetlands areas along the Sacramento once totaled more than ; today only about remains. Much of this consists of restored stretches and artificially constructed wetlands. Levee construction has prevented the river from changing course during winter and spring floods, which was crucial to the renewal of existing wetlands and the creation of new ones. Since the late 19th century the river has been mostly locked in a fixed channel, which once could shift hundreds of feet or even several miles in a year because of floods. In 2010, about of the river's riparian forests are undergoing active restoration. UC Davis initiated a project known as The Nigiri Project which takes place under the Yolo Bypass in the rice field floodplains adjacent to the Sacramento River. The name comes from a form of Japanese sushi which contains a slice of fish on top of a compressed wedge of vinegared rice, therefore a relationship with rice and fish. Salmon migrate from the Central Valley rivers to the ocean where they increase in size for one to three years then return to rivers to spawn, if a young fish is more large when they enter the ocean, they will have more of