Type: report

Article abstract: INTRODUCTION The City of San Diego (City) is required to implement a brown-headed cowbird ( Molothrus ater, Cowbird) trapping program at Kumeyaay Lake as requi red by the Biological Opinion issued for construction of the Kumeyaay Campground . The focus trapping area for the program in 2003 was Kumeyaay Lake. Santee, San Diego County, California. Varanus Biological Services (Varanus) placed traps adjacent to nati ve riparian habitats (Southern Willow Riparian Forest and Southern Cottonwood-willow Riparian Forest) in the vicinity of the lake. Since the early 1980's, Cowbird trapping has been an important component of recovery measures in suppo1t ofrecovery of the federally endangered least Bell's vireo (see Morrison et al. 1999), which is a small. insectivorous migratory songbird. The breeding range of the least Bell's vireo is southern California including portions of the California deserts. Least Bell's vireos winter in Baja California. Mex ico and typically migrate northward to their breeding grounds in southern Cal ifornia between mid-March and mid-April and occasionally as late as early May. During the breeding season the least Bell's vireo inhabits an assortment of riparian habitats. Dense low growing thickets of willows (Salix spp.), mule fat (Baccharis salicif olia) , California blackberry ( Rubus ursinus), Douglas' mugwort (Artemisia douglasiana) or other similar species are essential components of the habitat. An overstory composed of willows. cottonwoods (Pop11!11s fi'emo ntii), and/or sycamores ( Platanus racemosa) is often present . Other nesting habitats are dense patches of herbaceous understory in Coast Live Oak Riparian Forest and Coast Live Oak Woodland (W. Haas pers. obs), and occasionally patches of non-native habitat (P. Famolaro pers. com.) that now commonly form intricate mosaics with native habitats throughout the current breeding range of this species. Once widespread through much of California. the breedi ng range of the Least Bell 's Vireo extended from the northern edge of the Sacramento Valley of California to no1ihern Baja California. Least Bell's vireo population numbers declined dramatically between the l 940's and mid-1980's (Franzreb 1989). Two factors are primarily responsible for the decline and hence the endangered status of the Least Bell 's Vireo: habitat loss resulting from eradication, fragmentation, and degradation of riparian habitats; and brood parasit ism by Cowbird s (U. S. Fish and Wildlife Ser