| The leastBell's Vireo (Vireo bellii pusillus) is a small, grey bird listed on theFederal Endangered Species List. Once common in riparian habitats, by 1987the number of breeding pairs of least Bell's Vireo has declined to only300. The vireo is facing threat from two main sources: thedegredation ofthe riparian habitats vireo's call home, and increasing populations ofparasitic cowbirds.
The Brown-headed cowbird is a brood parasite of the vireo. A brood parasitelays eggs in other birds' nests. The hatchlings of the cowbird hatch earlier,and are larger and more aggressive than vireo hatchlings. Thus, the parentvireo spends most of it's time feeding the parasite and less food is availablefor its own offspring. When cowbirds are present, vireo nests rarely producefledged young. Brown-headed Cowbirds were originally associated with Bison on the GreatPlains, feeding on insects associated with the grazing animals. With theintroduction of wide-spread agriculture in the United States, cowbirdscame to associate themselves with grazing cattle. This has allowed thenumbers of cowbirds to explode, and the range of cowbird habitat to expandgreatly. The destruction and fragmentation of riparian habitat, and themovement of agriculture into close proximity with riparian areas has allowedthe cowbird to access riparian and woodland species that were previouslyunparasitized by cowbirds.
The Santa Ana Sucker (Catostomus santaanae) is a fish that was commonin streams of the Santa AnaWatershed andother rivers of Southern California asrecently as the 1970's. It is nowrarely found in theSanta Ana and San Gabriel Rivers, and has all but disappearedfrom other areas where it was once common. Because of the marked declinein the numbers of these fish, the U.S. Fish and Wildlife Service recentlylisted the Santa Ana Sucker as Threatened under the Endangered SpeciesAct.
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