MARCH 1998 Restoration Ecology Vol. 6 No. 1, pp. 75–82 75 © 1998 Society for Ecological Restoration Use of Restored Riparian Habitat by the Endangered Least Bell’s Vireo (Vireo bellii pusillus ) Barbara E. Kus Abstract A primary objective of riparian restoration in Califor- nia is the creation of habitat for endangered species. Four restoration sites in San Diego County were mon- itored between 1989 and 1993 and evaluated for their suitability as nesting habitat for Vireo bellii pusillus (Least Bell’s Vireo), a state and federally endangered obligate riparian breeder. Vegetation structure at each site was quantified annually and compared to a model of canopy architecture derived from Least Bell’s Vireo territories in natural habitat. Vireo use of restored habitat was documented through systematic surveys and nest monitoring. By 1993, only one site in its en- tirety met the habitat suitability criteria of the model, but portions of each site during all years did so. Dif- ferences between sites in the time required to develop suitable habitat—well-developed layered vegetation from the ground to under 8m in height)—were attrib- utable largely to variation in annual rainfall. Vireos visited restoration sites to forage as early as the first growing season, but they did not establish territories or nest there until at least part of the site supported suitable habitat as determined from the model. Place- ment of territories and nests coincided with patches of dense vegetation characteristic of natural nesting ar- eas. Occupation of restored sites was accelerated by the presence of adjacent mature riparian habitat, which afforded birds nest sites and/or foraging habitat lack- ing in the planted vegetation. Vireos nesting in restored habitat achieved success comparable to that of vireos nesting in surrounding natural habitat, and there was no evidence that productivity was reduced in created areas. These findings indicate that creating nesting habitat for this target species is feasible and suggest that the critical components of vireo nesting habitat have been captured in both the design and quantita- tive assessment of restoration sites. Introduction R iparian habitat in the southwestern United States has undergone serious declines during the last several decades, prompting efforts to create and restore woodlands throughout their historical distribution (An- derson & Ohmart 1982, 1985). One of the primary objec- tives of riparian restoration in southern California is to provide suitable habitat for endangered species. Although riparian habitat in this arid region is unparalleled in the diversity and abundance of wildlife it supports, thus warranting protection on this basis alone, it is the pres- ence of endangered species that provides the legal im- petus for mitigating habitat losses through restoration. Most of the riparian restoration in San Diego County has been driven by the requirement to mitigate for the loss of habitat supporting Vireo bellii pusillus (Least Bell’s Vireo), a state and federally endangered migra- tory songbird. This obligate riparian breeder was once abundant throughout California but declined in the face of widespread habitat destruction and parasitism by Molothrus ater (Brown-headed Cowbird). By 1986 the state population numbered just 300 pairs, roughly 80% of which occurred in San Diego County (U.S. Fish and Wildlife Service 1988; Regional Environmental Consult- ants 1989). Considerable advances have occurred over the last ten years in the technical aspects of the design and im- plementation of restoration sites (Baird 1989; Baird & Rieger 1989; Hendricks & Rieger 1989). But successful habitat restoration involves considerably more than the establishment of native vegetation in a natural setting and requires consideration of several perspectives. Bio- logically, the goal of restoration is to create self-sustain- ing ecosystems functionally equivalent to those being replaced (Jordan et al. 1987). Legally, the stated objec- tive of a restoration project may be more narrowly de- fined—for example, to create habitat capable of sup- porting a particular target species. Regulatory agencies are faced with the challenge of determining when the restoration has been successful in achieving its goal, as well as with identifying specific success criteria and methods for the quantitative measurement and evalua- tion of site performance. Implementing parties desire to achieve compliance with mitigation requirements through means that are not only economically efficient but 1 Department of Biology, San Diego State University, San Di- ego, CA 92182, U.S.A.