Type: report

Article abstract: Introduction In an effort to both protect nesting Southwestern Willow Flycatchers as well as mitigate for potential indirect impacts from off-site grazing on Forest Service lands, the U.S. Forest Service has funded a yeai-ly Brown-headed Cowbird trapping program along the upper San Luis Rey River since 1992. The purpose of this trapping program is to reduce the threat and impact of brood parasitism to the endangered Southwestern Willow Flycatcher (Empidona:x trailli extimus). The Brown-headed Cowbird (Molothrus ater) is an obligate brood parasite which lays its eggs in the nests of other bird species and is dependent upon the host to incubate their eggs and rear their young. Averaging 6"-7" in length, the Brown-headed Cowbird is a medium sized songbird with sexually dimorphic plumage. Adult males are dark brown to gloss black with a brown head and neck. Females are slightly smaller than males and dull tan to light brown with indistinct streaking on the breast. Originally restricted to the midwest region of North America, the Brown-headed Cowbird expanded in both range and abundance following the settlement and alteration of natural habitats, particularly with the increase in agriculture and livestock production. Reaching California in the late 1800's, this species was first documented breeding in San Diego County in 1915 (Unit 1984), and had become well established within southern California by the 1930's (Rothstein 1994, Willett 1933). Songbird species or populations which had not evolved with the cowbird and have no experience with parasitism may be subject to significantly reduced reproductive success. Brood parasitism combined with other impacts, such as habitat loss and fragmentation can lead to declines in songbird species, especially those with an already limited population and distribution. Cowbird trapping has proven to be an effective method in the conservation efforts of sensitive- . songbird populations throughout the United States, and was initially utilized in the recovery efforts of the Kirtland's Warbler in Michigan (Mayfield 1977). Subsequently, cowbird trapping has become an important tool in the management of several other sensitive songbird species, including the Black-capped Vireo, Least Bell's Vireo, and Southwestern Willow Flycatcher.