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4.7 BIOLOGICAL RESOURCES Wildlife The only species that breeds on the project site and is protected by law under the 1984 California Endangered Species Act is the Swainson's hawk. Because its presence has a major bearing on assessing the impacts which could result from construction of the proposed Buckeye Ranch project, a sound understanding of its biology and the current guidelines which govern its protection is necessary for a thorough evaluation of this situation. 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,and then in late August and September migrates back to Argentina and other neighboring countries for the fall and winter (U.S. Fish and Wildlife Service, 1986). Many factors have been postulated as possible causes for the declining population of Swainson's hawks in California. These include incompatible agricultural crops for the production and/or capture of prey (Bloom, 1980),grazing pressure (Detrich, 1986),predation on eggs and nestlings (U.S.Fish and Wildlife Service, 1986),depredation by humans on the wintering grounds (Bloom, 1980),pesticide use (Bloom, 1980;Detrich, 1986),the loss of habitat through land use conversions(California Department of Fish and Game, 1990),and direct competition with the more aggressive and abundant red-tailed hawk(Craighead and Craighead, 1956). Although the exact combination of factors responsible for its decline may never be known,there is no debate concerningthe fact that Swainson's hawk numbers have dropped drastically during the past century in California. Historically,this state's Swainson's hawk population may have exceeded 17,000 breeding pairs (Bloom, 1980). However,current population estimates average about 550 pairs for the entire state and 280 pairs for the Central Valley (California Department of Fish and Game, 1990). 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 pray. Major prey items 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 4.7-20