The least Bell's vireo (Vireo bellii pusillus) is a small, grey bird listed on the Federal Endangered Species List. Once common in riparian habitats, by 1987 the number of breeding pairs of least Bell's Vireo has declined to only 300. The vireo is facing threats from two main sources: the degradation of the riparian habitats vireo's call home, and increasing populations of parasitic cowbirds.

The Brown-headed cowbird is a brood parasite of the vireo. A brood parasite lays eggs in other birds' nests. The hatchlings of the cowbird hatch earlier, and are larger and more aggressive than vireo hatchlings. Thus, the parent vireo spends most of it's time feeding the parasite and less food is available for its own offspring. When cowbirds are present, vireo nests rarely produce fledged young.   Brown-headed Cowbirds were originally associated with Bison on the Great Plains, feeding on insects associated with the grazing animals. With the introduction of wide-spread agriculture in the United States, cowbirds came to associate themselves with grazing cattle. This has allowed the numbers of cowbirds to explode, and the range of cowbird habitat to expand greatly. The destruction and fragmentation of riparian habitat, and the movement of agriculture into close proximity with riparian areas has allowed the cowbird to access riparian and woodland species that were previously unparasitized by cowbirds.

HABITAT LOSS

The least Bell's Vireo is listed on the Federal Endangered Species List. The vireo's preferred habitat is the lower portions of riparian growth in the vicinity of water. The destruction of this habitat in many areas, combined with habitat degredation and the introduction of species unsuitable for vireo nests (especially Arundo donax), have been a major factor in the decline of the species.

The southwestern willow flycatcher (Empidonax traillii extimus) is a smaller bird currently recognized by both the Federal Government and the State of California as endangered. Once common, in 1987 only 300-500 nesting pairs remained. Since that time, virtually all monitored nesting groups have continued to decline. Habitat loss, combined with increased predation by cowbirds has been the primarily responsible for this decline.

The riparian (streamside) vegetation where the southwestern willow flycatcher nests has been degraded, and much of it has been destroyed. Removal of vegetation to clear land for agriculture, the invasion of non-native species such as arundo and saltcedar, and the alteration of hydrology by humans have all contributed to the loss of riparian vegetation. The Santa Ana Watershed is dedicated to the eradication of arundo and saltcedar throughout the watershed to preserve the riparian habitat for these species.

The Santa Ana Sucker is a small fish that was common in the Santa Ana Watershed. Changes in the Santa Ana Sucker's habitat, especially through water diversions, stream channelizations, erosion, and pollution, have been the prime cause in the decline of the species. Efforts are underway to preserve the Santa Ana Sucker. The Riverside-Corona RCD is in the process of creating an artificial stream to aid in the recovery of the fish to the watershed.