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APPENDIX I - BIOLOGICAL RESOURCES The actual home range area over which foraging occurs also varies greatly with land use. Studies recorded maximum foraging range in Wyoming as varying between 180 and 1,056 acres (Craighead and Craighead, 1956). In Central California, a range has been recorded from 30 to 16,000 acres (Estep, 1989). Studies have also found a direct correlation between foraging home range and cultivated land (Bechard, 1982). Where the cultivated land occupied more of the acreage in an area, the foraging area was consistently larger. Perhaps the most pertinent current estimate of foraging requirements are those given in the 1990 California Department of Fish and Game(CDFG)Mitigation Guidelines for Swainson's Hawk in the Central Valley of California(Appendix F). This document states that the minimum acreage needed to support a nesting pair is 1,200 acres and that all acreage within a 10-mile radius of an active nest is considered potential foraging habitat. 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 (Buteo jamenensis) (Craighead and Craighead, 1956). Although the exact combination of factors responsible for its decline may never be known, there is no debate concerning the fact that Swainson hawk numbers have dropped drastically during the past century in California. Historically,this state's Swainson's hawk population may have exceeded 17,OOObreeding 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). As previously stated, no counts or observations of Swainson's hawks were possible during the November 1991 survey period because they had long since migrated to their southern wintering grounds. Two reports show that Swainson's hawks use Brovelli woods. One is the Brovelli Woods Raptor Survey which was conducted for the applicant(Guse, 1990). This survey took place only over one-half of a full daylight period in late June and three possible nest sites were reported (Figure 4.7-2). The Habitat Conservation Plan for the Swainson's Hawk in San Joaquin County(Jones & Stokes Associates, 1990) reports two nesting territories for the project site and places all 25 nesting territories in the Dry Creek Swainson's Hawk subpopulation within CDFG's estimated average foraging radius of 10 miles of the site (Figures 4.7-8 and 4.7-9). Given this high density of nest sites in this subpopulation unit, Brovelli Woods could most likely support at least four and possibly more nesting pairs. Although these reports clearly show the presence of nesting Swainson's hawk pairs on the project site, there is no information as to how much these or additional pairs may use the grassland, oak savanna and dry Tracy Lakes basins as foraging habitat. An intensive spring/summer study would provide this information prior to consultation with California Department of Fish and Game Region 2 staff. 15