The Invasive Brown-Headed Cowbird
Endangered Species
The invasive brown-headed cowbird's parasitic behavior has led to the endangerment of two of California's native bird species. Cowbird parasitism reduces host species' productivity because cowbird chicks have an advantage in competition with host chicks, cowbird eggs often break host eggs when laid, female cowbirds often remove at least one host egg from nest, and cowbird eggs have a typically shorter incubation period than host eggs.
The Least Bell's Vireo
The Least Bell's Vireo is found in a riparian habitat in southern California, and its primary food source is insects. Historically, 80% of the southern California population was found in San Joaquin, Sacramento, Santa Clara, and Owens valleys. In 1980 it was listed as a state endangered species, and by 1986 it was listed as a federally endangered species as the population was reduced to less than 25% its original size. Other Bell Vireo subspecies populations stabilized between 1966 and 2014, except for the Least Bell's Vireo subspecies in California. In California cowbird parasitism is considered one of the major factors for Least Bell Vireo population decline.
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The Southwestern Willow Flycatcher
The southwestern willow flycatcher is also found in riparian habitats in southern California, and feeds primarily on small flying insects and contributes to the health of trees and forests. It was listed as a federally endangered species in 1995, and between 1996 and 2014 the population has decreased by 51%. Agricultural areas and livestock grazing has increased since previous years, and with it cowbird populations have increased as well, posing a greater threat to the southwestern willow flycatcher.