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APPENDIX I - BIOLOGICAL RESOURCES Wildlife Swainson's Hawk (Buteo swainsoni) State Listed: Threatened;' No Federal Listing The Swainson's hawk is a medium-size buteo or soaring hawk with long,pointed wings and a square tail. Unlike most hawks, it exhibits several color phases ranging from light through rufous, to very dark plumage. It is also unique among California raptors in that it migrates to the Central Valley from South America in late March and early April to nest and raise its young. In late August and September this species returns to Argentina and other neighboring countries for the fall and winter (U.S. Fish and Wildlife Service, 1986). Swainson's hawk is monogamous, and pairs remain together year after year until one dies. Nesting begins well after arrival in Central California,usually in May. Incubation lasts from 28 to 35 days, and nestlings are fledged at between four and eight weeks of age. Nest sites are solitary trees or small groves adjacent to agricultural fields or open grassland (Bloom, 1984). Many of the nesting areas are associated with riparian woodland. Schlorff and Bloom (1984) report that 82 percent of nests in their study were located within one mile of riparian forests. Estep (1989) confirms this finding with an estimate of 78 percent nesting in riparian areas. Nests are usually located near the tops of tall trees, sometimes up to 90 feet above the ground (Mallette and Gould, 1978). Nest locations are normally within easy flying distance to agricultural fields with abundant rodent prey. A recent report indicates that the Swainson's hawk is extremely refractory to human presence and activities within several hundred feet of the nest site, and will often desert the nest when such dis- turbance occurs (Estep. 1989). This behavior is possibly due to the fact that it endures heavy harass- ment by humans at its South American wintering grounds. The Department of Fish and Game currently recommends large buffer zones around any nest site because of the hawk's reaction to humans. Major prey sources include rodents (California meadow voles, pocket gophers, California ground squirrels), birds (meadow larks, mourning doves,young ring-necked pheasants), and larger insects such as grasshoppers and Jerusalem crickets. Foraging range depends on both the abundance and availability of prey. Thus, a given piece of land could have a very high mouse population,but if the vegetative cover precluded detection from an aerial perch,then the site would be of no value to this or any other soaring hawk species. Grassland areas that are moderately grazed may actually be better foraging sites for soaring hawks than ungrazed land, where dense grass and forb stands would mask rodent movement. ' T = Threatened. Plants or animals likely to become endangered in the foreseeable future in the absence of protection action(s). 14