Characteristics of Least Bell's Vireo Nest Sites Along

Due primarily to alteration ofriparian vegetation and nest parasitism by brown-headed cowbirds (Molothrus ater). the least Bell's vireo (Vireo bellii pusillus) has under­ gone a tremendous decline in range and numbers since the 1920's. In 1987. we sampled vegetation at 32 nest sites to characterize nesting habitat ofleast Bell's vireos in the Santa Ynez River drainage. Most next (59.4%) were located in willows (Salix spp.) or mugwort (Artemisia douglasiana) at heights ofless than 1 m. Vireos selected sites with relatively dense vegetative cover in the vicinity of the nests. Herba­ ceous species and dead plant material comprised much ofthe nesting cover. The least Bell's vireo nests in California and north­ western Baja California, and winters in southern Baja California. It is one of four recognized subspecies of Bell's vireo (Vireo bellii)(American Ornithologists' Union 1983). Nests are usually constructed in dense, willow­ dominated riparian vegetation within 3 m of the ground. Although formerly a common to abundant breeding species in the Central Valley and other low-elevation riparian zones in California, the least Bell's vireo has undergone a dramatic decline in abundance and distri­ bution. Despite a substantial decrease in numbers that began as early as the 1920s, this species was still widely distributed within California in the 1940s, extending northward to Red Bluff, Tehama County (Grinnell and Miller 1944). Since that time, the number and breed­ ing range of least Bell's vireos have steadily decreased, with all northern California populations believed to be extirpated by 1970 (Goldwasser and others 1980). The decline has been attributed primarily to: (1) alteration and destruction of riparian vegetation that comprises suitable breeding habitat; and (2) nest parasitism by brown-headed cowbirds. Because of this decline, the least Bell's vireo is a state-and federal-listed endangered species. The population in California in 1985 was estimated at approximately 300 pairs, based primarily on surveys conducted during the previous 12 years by Gaines (1974, 1977), Goldwasser (1978, 1981), Goldwasser and others (1980), and Gray and Greaves (1984). Nearly 20 percent of that total occurred along the Santa Ynez River in Santa Barbara County. Continued residential and industrial development in southern and central California has increased the de­ mand for water projects that could result in further al­ teration ofleast Bell's vireo nesting habitat. Information about this endangered species must be developed to re­ solve current and future conflicts between the demands of an increasing human population and habitat require­ ments of the least Bell's vireo. Such information will be necessary to mitigate adverse effects to vireos. The objective of this study was to characterize nest sites of least Bell's vireos and to describe trends in use of nesting habitat within the Santa Ynez River drainage. Study Area and Methods We sampled vegetation at 32 least Bell's vireo nest sites in 1987 along the Santa Ynez River in Santa Bar­ bara County (fig. 1). The study area was located ap­ proximately 10 km north of Santa Barbara and included the eastern end of Gibraltar Reservoir, a 3-km portion of the Santa Ynez River upstream from the reservoir, and Mono Creek from its confluence with the Santa Ynez River to the Mono Debris Basin. Approximately 240 ha of suitable vireo breeding habitat occurred in the study area. Riparian vegetation types included cottonwood forest, willow woodland, riparian scrub, and dry wash. Dominant overstory species were Fremont cottonwood (Populus fremontii), arroyo willow (Salix lasiolepis), and red willow (S. laevigata). Common species in a diverse understory included mugwort (Artemisia douglasiana) , mule fat (Baccharis salicifolia), and willow (Salix spp.) shrubs. Adjacent vegetation types were primarily chaparral, with smaller areas of oak woodland. Unlike most areas where least Bell's vireos nest in California, this study area was completely publicly owned and administered (United States Forest Service and the City of Santa Barbara). As such, little disturbance occurred in the study area and surrounding buffer areas. 1 Presented at the California Riparian Systems Conference; September 22-24, 1988; Davis, California. 2 Terrestrial Biologist, DaIIles & Moore, Goleta, California; Independent Consultant. Santa Barbara, California. 278 USDA Forest Service Gen. Tech. Rep. PSW-llO. 1989. :t:"'· '. L.~ •..• '.:".''''' .• ,