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Soil Investigations for Data Collection in the Delta Initial Study/Proposed Mitigated Negative Declaration 73 vegetation and will defend their territories from many species. Nesting occurs during June and July (Lowther et al. 1999). Yellow Warbler has a moderate potential to occur within the Study Area. Breeding is limited in the Central Valley in recent history, but the species has been observed in the Study Area during migration (Trochet et al. 2017). Implementation of Mitigation Measures MM AES-1 and MM BIO-1 and MM BIO-8 would reduce potential impacts to Yellow Warbler to: Less than Significant with Mitigation Incorporated. California Least Tern (Sternula antillarum browni) California Least Tern is listed as Endangered under CESA and FESA and is designated as Fully Protected under California Fish and Game Code. Least Tern is a small tern with narrow pointed wings, black crown, and white forehead. The historical breeding range of the California Least Tern extends along the Pacific Coast from approximately Moss Landing to the southern tip of Baja California (Grinnell and Miller 1944). However, since about 1970, colonies have been reported north to San Francisco Bay (USFWS 2006a). California Least Terns nest in loose colonies on barren or sparsely vegetated sandy or gravelly substrates above the high tide line along the coastline and in lagoons and bays of the California coast. Colonies occur near water that provides opportunities to forage for fish in shallow estuaries or lagoons (Thompson et al. 1997, CDFW 2005e, USFWS 2006a). Breeding occurs from mid-May through August (Massey and Atwood 1981, CDFW 2005e). California Least Tern has a low potential to occur within the Study Area. No suitable nesting habitat and no known nesting colonies are located within the Study Area, and foraging birds are rarely observed in the vicinity. No take of California Least Tern per California Fish and Game Code Section 3511 would occur due to the Proposed Project; however, CEQA considers potential effects beyond direct take of Fully Protected species. Implementation of Mitigation Measure MM BIO-1 would reduce potential impacts to California Least Tern to: Less than Significant with Mitigation Incorporated. Least Bell’s Vireo (Vireo bellii pusillus) Least Bell’s Vireo is listed as Endangered under FESA and CESA. Least Bell’s vireo is a small, drab songbird with brownish-gray plumage and two pale wingbars. The species’ historical distribution extended from coastal southern California through the San Joaquin and Sacramento Valleys as far north as Tehama County near Red Bluff (Kus 2002). The current breeding range is restricted to southern California, primarily San Diego County; however, recent nesting events at the San Joaquin River National Wildlife Refuge, along Putah Creek in Yolo Bypass, and Bradford Island in the central Delta indicate the species is attempting to recolonize the Central Valley. Least Bell's Vireo typically breeds in willow riparian forest supporting a dense, shrubby understory of