Type: report

Article abstract: INTRODUCTION The City of San Diego (City) is required to implement a brown-headed cowbird (Molothrus ater, BHCO) trapping program at Kumeyaay Lake as required by the Biological Opinion issued for construction of the Kumeyaay Campground. The focus trapping area for the program in 2001 was Kumeyaay Lake, Santee, San Diego County, California. Varanus Biological Services (Varanus) placed traps adjacent to native riparian habitats (Southern Willow Riparian Forest and Southern Cottonwood-willow Riparian Forest) adjacent to the lake. Cowbird trapping is conducted in support of recovery of the federally listed endangered least Bell's vireo ( Vireo bellii p usillus). Least Bell's vireos are small, insectivorous migratory songbirds of extremely limited range. Wintering in Baja California, Mexico, least Bell's vireos typically migrate northward to their breeding grounds in southern California between mid-March and mid-April. They breed almost exclusively in early succession willow-riparian habitats characterized by the presence of dense understory vegetation. Other nesting habitat includes dense patches of herbaceous understory in Coast Live Oak Riparian Forest and Coast Live Oak Woodland, and occasionally in patches of non-native habitat that now commonly form intricate mosaics with native habitats throughout the coastal range of this species. Once widespread in low-elevation riverine valleys as far north as Tehama County in northern California, least Bell's vireo population numbers declined dramatically between the 1940's and mid-l 980's. Two factors are primarily responsible for this species' demise and hence have contributed to its status as an endangered species: habitat loss resulting from eradication, fragmentation and degradation of southern California riparian corridors, and brood parasitism by BHCO. The explosive increase in BHCO population outside its historic range has accompanied the decline of riparian habitat. In southern California, the increase in BHCO has paralleled the decline in numerous other songbird populations including yellow warbler (Dendroica petechia ) and warbling vireo ( Vireo gilvus) in San Diego County (Unitt 1984). Brown-headed cowbirds were historically associated with buffalo and possibly antelope herds, and subsequently livestock in the Great Plains of the United States. It is hypothesized that brood parasitism in BHCO evolved to accommodate the species' reliance on benefits gained foraging among